WO2005119859A1

WO2005119859A1 - Indirect-bandgap semiconductor laser and method to fabricate same

Info

Publication number: WO2005119859A1
Application number: PCT/US2004/017225
Authority: WO
Inventors: Sylvain Cloutier; Jimmy Xu
Original assignee: Brown University
Current assignee: Brown University
Priority date: 2004-05-28
Filing date: 2004-05-28
Publication date: 2005-12-15
Anticipated expiration: 2006-11-28

Abstract

A solid state optical emitter has an active region having a layer of an indirect-bandgap semiconductor material. The layer is patterned to have nano-scale features and emits an optical signal characterized by an amplified spontaneous emission (ASE) spectrum, and is further characterized by having an emission peak within the ASE spectrum that has a plurality of characteristics that are consistent with a laser emission. In one embodiment the ASE spectrum is characterized by wavelengths in the range of about 1240nm to about 1290nm, and the emission peak is centered at about 1280nm.

Description

INDIRECT-BANDGAP SEMICONDUCTOR LASER AND METHOD TO FABRICATE SAME

TECHNICAL FIELD:

This invention relates generally to solid state optical emitter devices and, more specifically, relates to a silicon-based optical emitter device and, even more specifically, relates to a silicon-based laser device and to a method to manufacture a silicon-based laser device.

BACKGROUND:

Although highly desirable, silicon laser devices have not been available, nor have they been thought feasible. This general belief is rooted in the fundamental limitation that crystalline silicon is an indirect-bandgap semiconductor, as opposed to direct-bandgap materials, such as GaAs, which are commonly used in semiconductor-based optical emitters due to their high efficiency. In indirect-bandgap semiconductor materials, such as silicon, radiative recombination of electrons and holes is highly unlikely due to the large mismatch of momentum between the available electron and hole states. Despite this naturally-imposed fundamental limitation on light emission in silicon, attempts have been made to make silicon a more efficient light emitter. Some of these efforts that are directed towards enhancing silicon light emission attempt to achieve this goal via doping (see L.Pavesi, A review of the various efforts to a silicon laser, Photonics Packaging and Integration III, R.A.Heyler, D.J.Robbins and G.E.Jabbour, Ed., Proceedings of SPIE), or improved external coupling of the emitted photons (see M. A.Green, J.Zhao, A.Wang, P. J.Reece, M.Gal, Nature 412, 805 (2001)), or defect-states or surface recombination (see M.P.Stewart, J.M.Buriak, Jour, of the Am. Chem. Soc. 123, 7821 (2001), as well as O.Bisi, S.Ossicini, L.Pavesi, Surf. Sci. Rep. 38, 1 (2000)), or quantum-confinement effects (see, for example, W.L.Wilson, P.F.Szajowski, L.E.Brus, Science 262, 1242 (1993); Z.H.Lu, D.J.Lockwood, J.M.Baribeau, Nature 378, 258 (1995); L.Pavesi, L.Dal Negro, C.Mazzoleni, G.Franzo, F.Priolo, Nature 408, 440 (2000); J.Ruan, P.M.Fauchet, L.Dal Negro, M., L.Pavesi, Appl. Phys. Lett. 83, 5479 (2003); and A.G.Cullis, L.T.Canham, Nature 353, 335 (1991)). Porous silicon is one well-known example of this prior research, wherein a large surface-to-volume ratio and/or highly emissive surface states are created by severe chemical impregnation and material modification of the surface layer (see M.P.Stewart, J.M.Buriak, Jour, of the Am. Chem. Soc. 123, 7821 (2001); A.G.Cullis, L.T.Canham, Nature 353, 335 (1991); A.G.Cullis, L.T.Canham, P.D.J.Calcott, Jour, of Appl. Phys. 82, 910 (1997); and M.V.Wolkin, J.Jorne, P.M.Fauchet, Phys. Rev.Lett. 82, 197 (1999)). The emission due to nano-scale quantum confinement and related techniques, such as porous silicon materials, can be characterized by its sub-bandgap emission.

While optical gain enhancement has been achieved to varying degrees in these prior efforts, and in silicon nano-particles, no lasing action has been observed. This is not unexpected, however, given that in these systems an increase in optical gain is likely to be more than offset by an even greater increase in internal absorption and scattering by the random features.

Thus, while some progress has been reported in the generation of light emission in silicon, prior to this invention lasing has not been observed, nor was lasing believed possible due to the indirect-bandgap of silicon.

SUMMARY OF THE PREFERRED EMBODIMENTS

The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings.

In one aspect this invention provides a solid state optical emitter device having an active region comprised of a layer of semiconductor material. The layer is patterned and emits, when pumped, an optical signal having a plurality of characteristics that are consistent with a laser emission. The layer is patterned so as to increase a surface-to-volume ratio and a concentration of bound excitons in the layer to create a spatially confined exciton population that enhances recombination proportionally to nano-patterning-induced surface enhancement. The plurality of characteristics that are consistent with a laser emission include threshold behavior, a linear power evolution, linewidth narrowing and a polarized emission, indirect-bandgap. The layer of semiconductor material may be a direct-bandgap or an indirect-bandgap semiconductor material.

In one presently preferred embodiment the layer of indirect-bandgap semiconductor material is comprised of intrinsic silicon, and the optical signal has a wavelength of about 1280nm.

In one embodiment the layer is patterned to have a periodic array of uniform and ordered nano-scale holes, while in another embodiment the layer is patterned to have an array of upstanding nano-scale features, such as posts or pillars. The combination of the feature size with the regularity and uniformity of the feature array cooperate to provide the significant enhancement in optical properties that are observed in an optical emitter constructed using indirect-bandgap semiconductor material

Also described is a method to fabricate a solid state laser having an active region. The method includes providing a layer comprised of an indirect-bandgap semiconductor material, and patterning the layer so as to increase a surface-to- volume ratio and a concentration of bound excitons in the layer to create a substantially two dimensional confined exciton-liquid that enhances free-exciton recombination.

In a preferred, yet non-limiting, embodiment of the method the step of providing includes providing a silicon-on-insulator (SOI) wafer having a silicon epilayer; thinning the silicon epilayer; applying a patterning mask to a top surface of the silicon epilayer; and etching the silicon epilayer through the mask so as to pattern the silicon epilayer. In the preferred embodiment thinning results in a silicon epilayer thickness of less than about 1 OOnm. In one embodiment etching yields an array of throughholes in the silicon epilayer having diameters of the order of tens of nanometers, while in another embodiment etching yields an array of upstanding features having diameters of tens of nanometers.

In one preferred embodiment of the method applying a patterning mask applies a mask fabricated by a nano-scale self assembly process, such as a mask fabricated by an anodic aluminum oxide (AAO) fabrication process.

Also disclosed herein is a solid state optical emitter having an active region comprised of a layer comprised of an indirect-bandgap semiconductor material. The layer is patterned to have nano-scale features and emits an optical signal characterized by an amplified spontaneous emission (ASE) spectrum, and further characterized by having an emission peak within the ASE spectrum that has a plurality of characteristics that are consistent with a laser emission. In a preferred, yet non-limiting embodiment the ASE spectrum is characterized by wavelengths in the range of about 1240nm to about 1290nm, and the emission peak is centered at about 1280nm.

Disclosed herein is a device capable of generating laser action at a wavelength of about 1280nm in a periodic nano-patterned Si superstructure formed in a crystalline silicon-on-insulator wafer. Threshold behavior, linear power-evolution, linewidth narrowing and polarized emission, indicative of the occurrence of laser action, are observed at temperatures below 40K, demonstrating that under controlled and uniform nano-scale modification of silicon, the optical gain can be made sufficiently large, and the material losses made sufficiently small, so as to enable lasing to occur. BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects of these teachings are made more evident in the following Detailed Description of the Preferred Embodiments, when read in conjunction with the attached Drawing Figures, wherein:

Figs. 1 A- 1 D, collectively referred to as Fig. 1 , illustrate a nano-patterned silicon epilayer of a SOI substrate, a fabrication mask and spectral emission;

Fig. 2 is a graph that depicts the evolution of a 1279nm lasing-peak intensity as a function of the incident pump power density at 40, 30, 15 and 10K;

Fig. 3 is a graph that shows the emission spectra through a broadband high-extinction polarizer with its polarization axis set parallel and peφendicular with respect to the surface of the sample;

Figs. 4A-4D, collectively referred to as Fig. 4, illustrate the edge emission of the nano- patterned and unpatterned (reference) SOI samples;

Fig. 5 shows a process for fabricating the nano-patterned SOI sample, in accordance with one embodiment of this invention;

Figs. 6A and 6B, collectively referred to as Fig. 6, illustrate a silicon nano-pillar array, and a silicon nano-dot array, respectively, in accordance with further embodiments of this invention;

Figs. 7A, 7B and 7C, collectively referred to as Fig. 7, illustrate electrically pumped nano-patterned Si laser device embodiments further in accordance with this invention; and

Figs. 8A and 8B, collectively referred to as Fig. 8, illustrate embodiments of doped, nano-patterned silicon optical emitters.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 1 illustrates an embodiment of a nano-patterned silicon layer that sits atop a SOI substrate and a resulting spectral emission. More specifically, Fig. 1A illustrates a 45-degree elevational enlarged (scanning-electron microscope (SEM)) view of a nano-patterned silicon-on-insulator sample 10. As a non-limiting example, samples used exhibited a 60nm-thick layer of silicon <100> atop a 3μm thick layer of silicon-oxide on a silicon substrate (a SOI substrate or wafer). The 60nm-thick Si layer 10A was processed to form a regular array of through-holes, also referred to as pores 12. The average diameter of each of the pores 12 is 55nm and the spacing between the pores 12 is l lOnm center-to-center. Fig. IB is a SEM top-view of an anodic aluminum oxide (AAO) highly ordered nano-pore array mask 20 used to pattern the Si layer 10A shown in Fig. 1 A. Fig. 1 C illustrates edge-emission spectra at T=70K, 50K, 30K and 1 OK, while Fig. 1 D shows the continuous edge-emission spectral evolution with temperature. While the amplified spontaneous emission (ASE) background emission was found to persist up to 80K, in this particular sample al279nm laser-peak emission appears only at 40K and below. As is shown in Fig. 1A, the all-silicon mesoscopic structure of the sample 10 was formed, in this embodiment, by etching a highly ordered array of nano-scale sized holes (also referred to herein as nano anti-dots and as nano-pores, or simply as the pores 12) into the crystalline, device grade Si epilayer 10 of the SOI wafer. The resulting nano- scale patterned Si layer 10A may be referred to as a superlattice. A novel fabrication technique, described in further detail below, was used for the formation of the micro- patterned silicon superstructure in a commercially available electronic-grade SOI wafer. This technique permits controlled nano-patterning over a relatively large area, while maintaining a simple and clean fabrication process.

As employed herein, nano-scale features are assumed to be features having sub- micrometer dimensions, such as diameters, thicknesses and/or spacings.

Referring to Fig. 5A, a commercial grade SOI wafer 50 as originally obtained had a 205nm-thick silicon <100> layer 52 atop a 3μm substantially non-electrically conductive dielectric (oxide) layer 54 on a silicon <100> substrate 54. The silicon <100> layer 52 is one that may be referred to as intrinsic, and in this embodiment is not intentionally doped. Referring to Fig. 5B the top silicon layer 52 was thermal-oxidized and then thinned to less than, for example, lOOnm via wet-etching of the resulting oxide layer in a 1 :6 HF:H₂O solution. The final thickness of the thinned top silicon layer 52A was measured using a Dektak-3 profilometer to be, in this non-limiting example, 60nm. An un-patterned piece of the thinned SOI sample 40 was set aside and used for emission reference and comparison, as discussed below in reference to Figs. 4B, 4C and 4D. A 10 mm by 1.5 mm portion of the patterned region was cleaved for performing the edge-emission measurements.

An important element of the fabrication process is the use of the nano-pore array AAO membrane as the etch mask 20. The AAO etch mask 20 was fabricated in a two-step anodization process in which uniform nano-pores form and self-organize into a highly-ordered hexagonal array. As is known in the art, the pore diameter and spacing can be controllably changed by varying the anodization conditions. Reference in this regard can be made to J.Liang, H.Chik, A.Yin, J.Xu, Jour.of Appl. Phys. 91, 2544 (2002), to A.P.Li, F.Mϋller, A.Birner, K.Nielsch, U.Gόsele, Jour, of Appl. Phys. 84, 6023 (1998), and to G.E. Thomson, Thin Solid Films 297, 192 (1997) and references therein. Reference can also be made to F. Li, L. Zhang, and R.M. Metzger, "On the Growth of Highly Ordered Pores in Anodized Aluminum Oxide", Chem. Mater. 10, 2470-2480 (1998), which describes a detailed protocol suitable for the fabrication of the AAO etch mask 20. A detailed protocol for the lift-off of the AAO and its use as a mask for nano-pattering can be found in the above-cited J.Liang, H.Chik, A.Yin, J.Xu, Jour.of Appl. Phys. 91, 2544 (2002). In the example of the AAO mask 20 shown in Fig. IB, the AAO membrane was grown to 700nm thick, and was subsequently placed on top of the 60nm-thick silicon layer 52A of the SOI substrate while immersed in water (Fig. 5C). The nano-pore array pattern was then transferred into the 60nm-thick silicon layer 52A using a chlorine-based reactive-ion etching (RIE) process. The AAO is an effective mask in the RIE process and permits the formation of clean, deep and straight nano-holes into the silicon layer 52A. Afterwards, the AAO mask 20 is removed, yielding the micro- patterned Si sample 10 as shown in Fig. 1A. In the preferred, but not limiting embodiment the pores 12 are made completely through the thinned Si layer 52A, and the underlying oxide layer 54 functions as an etch stop.

An annealing and/or planarization process may also be performed to reduce any resulting surface roughness, thereby also reducing scattering losses. One suitable and non-limiting process for reducing surface roughness is ICP etching, a method of high aspect ratio material removal by plasma etching using an inductively-coupled RF source. Another suitable and non-limiting process for reducing surface roughness is a post-etch anneal in a H-N gas mixture (at about a 5/95 ratio) at elevated temperatures. By thus reducing scattering losses, the temperature at which lasing occurs can be increased.

Post-processing, such as by using thermal oxidation and HF etching, can also be used to further thin the Si layer 10 and/or to widen the diameter of the pores 12, thereby permitting post-fabrication structural modifications.

For lasing to occur, it is important that the modal loss be less than the modal gain. The large degree of uniformity achieved by the foregoing fabrication process is advantageous in maintaining a low optical loss in the sample 10. To minimize internal material loss an undoped crystalline SOI wafer 50 was selected, which resulted in the use of optical pumping. As is shown in Fig. 1 C, a continuous- wave (CW) 514.5nm Argon-laser beam 30 was focused onto a 200μm-diameter spot on the top-surface of the micro-patterned silicon sample 10 close to an edge, and the edge-emission 35 was collected. To further increase the optical gain and minimize the internal loss, the sample 10 was placed in a cryostat and cooled to temperatures below 77K. The edge-emission 35 of the sample 10 was collected through an optical window of the cryostat and focused on a 2mm- wide monochromator entrance-slit for spectral measurements.

Referring also Fig. ID, at cryogenic temperatures below 77K a broad ASE spectrum spanning from about 1240nm to about 1290nm was measured from the sample 10. Below 40K, a spectral peak emerged at about 1279nm. This spectral peak was found to grow in intensity while narrowing in width, from approximately 2.3nm to approximately 1.5nm, with decreasing temperature, a classic spectral behavior of lasing action. The apparent linewidth of the lasing peak at 1 OK was measured to be approximately 1.5nm, with a more precise measurement of its likely smaller actual value being limited by the monochromator's entrance-slit opening (2mm in this case). The continuous temperature-evolution of the emission spectrum shown in Fig. 1 D provides further details of the spectral behavior.

Fig. 2 shows the evolution of the 1279nm lasing-peak intensity as a function of the incident pump power-density at 40K, 3 OK, 15K and 1 OK. A linear power-evolution with a low-power threshold can be clearly observed, along with a decreasing threshold and an increasing slope with decreasing temperature.

Fig. 3 shows the emission spectra through a broadband high-extinction polarizer with its polarization axis set parallel and peφendicular with respect to the surface of the sample 10. sample's surface. The results show that the 1279nm laser-peak emission is strongly polarized parallel to the axis of the pores 12 (peφendicular to the surface silicon film), whereas the ASE background shows a very weak degree-of-polarization (DOP). The inset in Fig. 3 plots the normalized lasing-peak intensity as a function of the axis orientation of the polarizer. The strong DOP of the 1279nm emission peak, together with the weak DOP of the ASE background, clearly suggest the presence of coherent lasing emission.

Similar to prior findings (M.A.Green, J.Zhao, A.Wang, P.J.Reece, M.Gal, Nature 412, 805 (2001), and M.J.Chen et al., Appl. Phys. Lett. 84, 2163 (2004)), a significant enhancement of the free-exciton (FE) emission was observed at room temperature in the patterned silicon layers. The surface-patterning of a silicon surface is known to lead to significant emission enhancement from increased external coupling of light emission and absoφtion.

Fig. 4 A demonstrates that cooling the patterned silicon sample 10 to about 10K further increases the FE recombination-peak intensity by two orders of magnitude. A strong emission enhancement in the nano-patterned sample 10 remains over a large range of pumping power at this temperature, as shown in Fig. 4B. At 10K, well-known features in the emission spectral band extending from 1080nm to 1220nm are observed in the edge-emission from both the nano-patterned sample 10 and from the un-patterned reference sample 40, as shown in Fig. 4C. Emission peaks associated with the FE-recombination via transverse acoustic (TA), longitudinal and transverse optical (LO, TO), as well as two-phonon (2-phonon) coupling, were observed in both the nano-patterned sample 10 and the unpatterned sample 40. Reference in this regard may be had to Y.Pokrovskii, Phys. Stat. Sol. (a) 11, 385 (1972); M.A.Vouk, E.C.Lightowlers, Jour, of Phys. C 10, 3689 (1977); N.Drozdov, A.Fedotov, Jour, of Phys. 14, 12813 (2002); and L.V.Butov, C.W.Lai, A.L.Ivanov, A.C.Gossard, D.S.Chemla, Nature 417, 47 (2002). In addition to a significant emission-enhancement due to the nano-patterning of the surface of the sample 10, consistent with reports for nano-patterned silicon (see M.A.Green, J.Zhao, A.Wang, P.J.Reece, M.Gal, Nature 412, 805 (2001); and M.J.Chen et al., Appl. Phys. Lett. 84, 2163 (2004)) and reports for silicon nano-particle assembly (see M.J.Chen et al., Appl. Phys. Lett. 84, 2163 (2004) and L.Pavesi, L.Dal Negro, C.Mazzoleni, G.Franzo, F.Priolo, Nature 408, 440 (2000)), also observed was an enhanced bound-exciton (BE) peak at 1134nm (see N.Drozdov, A.Fedotov, Jour, of Phys. 14, 12813 (2002)). An emission band from 1140 to 1165nm was also observed exclusively from the nano-patterned sample 10. It is believed that this emission band can be attributed to exciton-condensation in so-called electron-hole droplets (EHD), see Y. Pokrovskii, Phys. Stat. Sol.(a) 11, 385 (1972).

Moreover, the broad ASE emission in the longer wavelength spectral band from 1240nm to 1290nm, shown in Fig.4D, was only observed from the highly-ordered silicon anti-dot array that characterizes the nano-patterned sample 10. Under the same conditions it was found that the reference unpatterned SOI sample 40, obtained from the same wafer as the nano-patterned sample 10, as described above, exhibited no measurable emission in this band. While a clear understanding of the exact origin of this broad ASE emission has yet to be established, its presence can be attributed at least in part to the use of crystalline, undoped silicon. It seems unlikely, however, that this long-wavelength emission-band can be directly attributed to quantum-size effects, since the effective electron wavelength is much smaller than the feature sizes in the nano-patterned sample 10. The long- wavelength emission band may be attributed to an increased concentration of bound excitons, and to their nucleation around the surface of the nano holes (the pores 12), thereby effectively creating a two dimensional (2D) confined exciton-liquid and increasing the FE recombination proportionally to the significantly increased surface-to-volume ratio (see Y.Pokrovskii, Phys. Stat. Sol. (a) 11, 385 (1972); N.Drozdov, A.Fedotov, Jour, of Phys. 14, 12813 (2002); and L.V.Butov, C.W.Lai, A.L.Ivanov, A.C.Gossard, D.S.Chemla, Nature 417, 47 (2002)). It can be noted that one effect of the strong localization of the excitons is an associated broadening or spreading of their k- vector value, resulting in relaxation of the constraint of the k-mismatch in the indirect-bandgap material (silicon in this case) and an increase in the probability for radiative recombination. In contrast to the porous silicon and the silicon nano-particle assembly used in the prior art, it is believed that the uniformity of the nano-hole array (as evidenced at least by the uniformity of the pore size and the center-to-center spacing) aids in condensing the bound exciton transitions to a narrow spectral range, thereby contributing to an increase in the peak gain, while maintaining the scattering loss at a minimum.

While the cleaved facets could provide a resonant cavity and the necessary feedback mechanism for lasing, the inventors are at present unsure whether this is the primary source of feedback, given the rather long cavity length and the largely absent side lasing modes. However, such a Fabry-Perot resonant-cavity is not the only possible feedback mechanism, even though it is the most common. For example, non-resonant feedback, "mirrorless" or "random" lasers (see RN.Ambartsumyan, N.G.Basov, P.G.Kryukov, V.S.Letokhov, Prog, in Quant. Elect. 1, 109 (1970)) are also known to be possible in structures where densely distributed sub-wavelength scatterers enable coherent backscattering, or so-called photon-localization in a gain medium, and thereby laser emission (see, as example, S.John, Phys. Rev. Lett. 58, 2486 (1987); S.John, Physics Today 44, 32 (1991); S.John, G.Pang, Phys. Rev. A 54, 3642 (1996); R.Dalichaouch, J.P.Armstrong, S.Schultz, P.M.Platzman, S.L.McCall, Nature 354, 53 (1991); and N.M.Lawandy, R.M.Balachandran, A.S.L. Gomes, E.Sauvain, Nature 368, 436 (1994), addendum: N.M.Lawandy and R.M.Balachandran, Nature 373, 204 (1995)). More recently, this type of laser system has been demonstrated in densely-packed direct- bandgap semiconductor nano-particle powders (see D.S.Wiersma, P.Bartoloni, A.Lagendijk, R.Righini, Nature 390, 671 (1997) and references therein; H.Cao et al., Phys. Rev. Lett.82, 2278 (1999); M.Kazes, D.Y.Lewis, Y.Ebenstein, T.Mokari, U.Banin, Adv. Mat. 14, 317 (2002); and S.F.Yu, C.Yuen, S.P.Lau, W.I.Park, G.C.Yi, Appl. Phys. Lett.84, 3241 (2004)).

Further in this regard, it is noted that a "photonic bandgap" feedback mechanism is an unlikely source of the observed emission that is consistent with laser action, as it can be shown that the period of the required nano-patterned array would be considerably larger than actually present in such a thin silicon film with an approximately 70% fill factor. The strong polarization shown in Fig. 3 is not only a test of coherence, but is also indicative of a possibly significant role of coherent-backscattering feedback caused by the vertically oriented pores 12, which provides a much longer path-length of lasing oscillation (parallel to the axes of the pores 12), than the ASE-background emission.

Thus, while the strongly polarized lasing peak versus the unpolarized ASE clearly suggests coherence (hence feedback), the exact nature of the feedback mechanism or mechanisms is, at this time, not fully defined. However, the fact that lasing action, manifesting the classic characteristics of threshold, linewidth narrowing, linear power evolution and coherence, is caused to occur in crystalline silicon indicates that a relatively simple, nano-scale but highly-uniform spatial modification offers the potential to greatly enhance light emission in silicon, while minimizing the optical loss. This also demonstrates that in the long (below bandgap) wavelength regime, the optical gain can be increased over the material absoφtion and scattering loss such that lasing action can be supported. While the observed lasing action described above occurred at cryogenic temperatures, further development and optimization of the structure is expected will enable improved performance and lasing to occur at higher temperatures.

Relatedly, it is within the scope of this invention to perform post- fabrication processing of the dot (Fig. 6) or anti-dot (Fig. 1) diameters and layer thicknesses to optimize the emission to absoφtion ratio(and carrier concentration). This can be accomplished, for example, through thermal oxidation followed by a wet etch of the Si in HF.

While described thus far in the context of optically-pumped laser device embodiments, the teachings of this invention can also be extended to electrically pumped laser embodiments. In addition, the surface of the silicon sample 10 can be nano-patterned using, instead of holes, a pedestal or pillar-type of pattern. For example, Figs. 6A and 6B illustrate a silicon nano-pillar array 60 of 30nm diameter, and an upstanding silicon nano-dot array 62, respectively, in accordance with further embodiments of this invention. In addition, Figs. 7 A, 7B and 7C illustrate electrically pumped nano-patterned Si laser device embodiments 70, 80 and 90, respectively, in accordance with further embodiments of this invention.

Fig. 7A shows a vertical current injection Si laser device 70 embodiment, wherein a Si active layer 72, that is nano-patterned in accordance with the embodiments of Figs. 1 or 6, is disposed between an n-type substrate 74 and a p-type cladding layer 76 (note that the n and p conductivity types could be reversed). Electrodes 74A and 76A are used to couple the vertical current injection Si laser device 70 to a source of electrical current. Thin oxide layers 75A and 75B are disposed between the Si active layer 72 and p-type cladding layer 76 and the n-type substrate 74. The top cladding layer 76 preferably has a larger bandgap than the silicon-based (or Si-Ge based) gain layer 72. An epitaxial regrowth process may be preferred to create the p-type cladding layer 76.

Fig. 7B shows a lateral current injection Si laser device 80 embodiment, wherein a Si active layer 82, that is nano-patterned in accordance with the embodiments of Figs. 1 or 6, is disposed laterally between an n-type layer 84 and a p-type layer 86, all of which are disposed on a semi-insulating substrate 88. In this case the electrodes 84A and 86A are used to couple the lateral current injection Si laser device 80 to a source of electrical current. An epitaxial regrowth process may be preferred to create an intrinsic cladding layer 82A that lies above the Si active layer 82.

It is noted that in Figs. 7A and 7B the layer 72/82 can be intrinsic (or undoped) assuming that layers 76/86 and 74/84 are respectively p-doped and n-doped. However, it is not necessary to use only undoped silicon when constructing optical emitters in accordance with this invention. One may, for example, overlap two layers 102, 104 of n-doped and p-doped silicon and pattern both, as shown in Fig. 8. In the embodiment of Fig. 8 A, having top-surface and bottom-surface electrodes 76A, 74A, the substrate 100A is conductive, while in the embodiment of Fig. 8B, having the top-surface electrodes 86A, 86B the substrate 100B is insulating. In such cases, the patterned (active) layer(s) are doped indirect-bandgap semiconductor material. However, while the devices illustrated in Figs. 7 A and 7B may be more efficient, Figs. 8 A and 8B show that it is possible to pattern doped silicon and use it as an active region for electrical pumping.

Fig. 7C shows the lateral current injection Si laser device 80 embodiment of Fig. 7B integrated with a MOSFET 90. The MOSFET 90 includes a FET gate electrode 92 disposed over a layer of gate oxide 94 between the n-type laser injection layer 84 and another n-type layer 96. Control of the gate voltage results in turning on and off the current flow through the MOSFET 90, and thus the turning on and off the current injection into the lateral current injection Si laser device 80. This embodiment can thus be particularly useful for modulating the emitted light for optical communication and other applications. For example, the modulated (on/off or otherwise modulated) optical emission at about 1280nm can be coupled into an optical fiber and transmitted to a remotely located optical receiver. Alternatively, the modulated optical emission at about 1280nm can be coupled into an on-chip optical conduit, such as a waveguide, and transmitted to an on-chip optical receiver. As can be appreciated, this embodiment of the invention is particularly attractive as it enables the integration of silicon-based photonics, including a silicon-based optical emitter, with silicon-based microelectronics in and on a common silicon substrate. It should be noted that the embodiments of Figs. 7A, 7B and 7C are also edge emitting devices, in the same manner as the optically-pumped embodiment shown in Fig. lC. Note further that due to the minimal absoφtion by the silicon of the emitted wavelengths, that absoφtive self-heating of the laser devices is small.

It should further be noted, however, that this invention can also be applied to vertical emission devices. For example, the construction shown in Fig. 7A could be fabricated to be a vertical emission device, where layer 72 would be p-doped, and where 76/76A would be a transparent electrode, such as a film of indium-tin-oxide (ITO). One may refer to the above-cited M.A.Green, J.Zhao, A.Wang, P.J.Reece, M.Gal, Nature 412, 805 (2001), although the device therein is non-lasing and operates by a different physical mechanism.

The foregoing description has provided by way of exemplary and non-limiting examples a full and informative description of the best method and apparatus presently contemplated by the inventors for carrying out the invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. As but some examples, the use of this invention is not limited to generating light of about 1280nm (i.e., light having wavelengths that falls in the optical communication wavelengths), as other wavelengths may be generated depending on the characteristics of the nano-patterning and thus, at least to some degree, to the characteristics (including the feature size, spacing and regularity) of the mask 20. Note in this regard that the pore diameter and the pore spacing can be controllably changed, i.e., the self-assembly characteristics of the AAO can be controlled, by varying the anodization conditions and/or by altering the self-adhesion to silicon, as is known in the art. However, in other embodiments of this invention the mask 20 may be fabricated using different self-assembling materials (bottom-up techniques), or the mask 20 may be fabricated in metal or some other suitable material using a photolithographic process (including e-beam lithography), or by direct laser writing and patterning of a suitable mask material (top-down techniques). In addition, and as was noted, the use of this invention is not restricted to cryogenic temperatures, as improvements in material processing and other developments are expected to further reduce optical losses, and thus increase the operating temperature at which lasing occurs. Further, another medium may be provided in the gap spaces (e.g., within the nano-pores 12) to at least one of increase the gain, or reduce scattering losses, or provide index matching. Note that the nano- patterned silicon can also be sandwiched in the gain medium if it has a lower index of refraction (although this would be less efficient but could still enhance the emission). Examples of suitable materials that can be employed to one or more of increase the gain, reduce the scattering loss, provide index matching, tunability or other functionality include, but are not limited to, polymer-dye systems (such as fluorescent dyes, azobenzene-dyes and laser-dyes), liquid crystals (tunability), polymer-liquid crystal systems, Polymer-Dispersed Liquid Crystals (PDLCs), Rare-Earth-doped glass (e.g., Erbium-doped glass, or Thulium-doped glass), chalcogenides glasses (for nonlinear- optical properties and/or tunability), non-linear crystals (for example, KDP, BBO) for harmonic generation or frequency conversion, and other semiconductor materials (for quantum-dot or heterostructures).

Also, while described above primarily in the context of silicon as the indirect-bandgap semiconductor material, this invention can be used to advantage with other indirect- bandgap semiconductor materials, such as germanium, and with alloys of indirect- bandgap materials, such as Si-Ge alloys.

The teachings of this invention can also be employed with direct-bandgap materials, such as Group III-V materials, to enhance their light output.

It should be noted that the use of undoped silicon in the optically pumped embodiment discussed above was desirable in order to reduce the round-trip loss and enable amplification. However, in the electrically-pumped laser embodiments discussed above it may be highly desirable to use doped silicon.

Further, the use of the silicon epilayer of a SOI wafer is not a limitation upon the practice of this invention, but was described as a convenient source of high-quality silicon within which to fabricate the laser device in accordance with this invention. In other embodiments a silicon layer may be formed by suitable epitaxial deposition processes, and used to fabricate the edge emitting or vertical emitting laser embodiments in accordance with this invention.

Further, the stacking of multiple layers of nano-patterned silicon, or the use of tri- dimensional patterning to increase the emission efficiency by surface-to-volume ratio or active-material density enhancement, are all within the scope of this invention.

Note further that in some embodiments it may be desirable to employ a combination of hole features and upstanding pillar or post-like features in the same optical emitter device.

However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention. j Furthermore, some of the features of the present invention could be used to advantage without the corresponding use of other features. As such, the foregoing description should be considered as merely illustrative of the principles of the present invention, and not in limitation thereof.

Claims

CLAIMSWhat is claimed is:

1. A solid state optical emitter device having an active region comprised of at least one layer of silicon, said layer of silicon being patterned and emitting when pumped an optical signal having a plurality of characteristics that are consistent with a laser emission.

2. A solid state optical emitter device as in claim 1 , where said plurality of characteristics that are consistent with a laser emission comprise threshold behavior, a linear power evolution, linewidth narrowing and a polarized emission.

3. A solid state optical emitter device as in claim 1, where said optical signal has a wavelength of about 1280nm.

4. A solid state optical emitter device as in claim 1 , where said layer of silicon is patterned to have at least one of an array of hole features and an array of upstanding features.

5. A solid state optical emitter device as in claim 5, further comprising a material provided in gap spaces of said features to at least one of increase the gain, reduce scattering losses, provide index matching, and provide tunability.

6. A solid state optical emitter device as in claim 1, where said layer of silicon has a thickness of less than about lOOnm and is patterned to have an array of throughholes, where said array of throughholes comprises holes having a diameter of about 55nm and a center-to-center spacing of about l lOnm, and where said optical signal has a wavelength of about 1280nm.

7. A solid state optical emitter device as in claim 1, where said layer of silicon is optically pumped.

8. A solid state optical emitter device as in claim 1, where said layer of silicon is electrically pumped in a vertical configuration.

9. A solid state optical emitter device as in claim 1, where said layer of silicon is electrically pumped in a lateral configuration.

10. A solid state optical emitter device as in claim 1, where said device is integrated on a substrate and said layer of silicon is electrically pumped via at least one transistor device that is integrated on the same substrate.

11. A solid state optical emitter device as in claim 1 , where said layer of silicon is not intentionally doped.

12. A solid state optical emitter device as in claim 1, where said device is operated at a cryogenic temperature.

13. A solid state optical emitter device having an active region comprised of a layer of indirect-bandgap semiconductor material, said layer being patterned and emitting when pumped an optical signal having a plurality of characteristics that are consistent with a laser emission, where said layer is patterned so as to increase a surface-to-volume ratio and a concentration of bound excitons in the layer to create a spatially confined exciton population that enhances free-exciton recombination.

14. A solid state optical emitter device as in claim 13, where said plurality of characteristics that are consistent with a laser emission comprise threshold behavior, a linear power evolution, linewidth narrowing and a polarized emission.

15. A solid state optical emitter device as in claim 13, where said layer of indirect- bandgap semiconductor material is comprised of intrinsic silicon, and where said optical signal has a wavelength of about 1280nm.

16. A solid state optical emitter device as in claim 13, where said layer is patterned to have an array of nano-scale holes.

17. A solid state optical emitter device as in claim 13, where said layer is patterned to have an array of upstanding nano-scale features.

18. A solid state optical emitter device as in claim 13, where said device is operated at a cryogenic temperature.

19. A method to fabricate a solid state laser having an active region, comprising:

providing a layer comprised of an indirect-bandgap semiconductor material; and

patterning the layer so as to increase a surface-to-volume ratio and a concentration of bound excitons in the layer to create a substantially two dimensional confined exciton-liquid that enhances free-exciton recombination.

20. A method as in claim 19, where providing comprises providing a silicon-on-insulator (SOI) wafer having a silicon epilayer; thinning the silicon epilayer; applying a patterning mask to a top surface of the silicon epilayer; and etching the silicon epilayer through the mask so as to pattern the silicon epilayer.

21. A method as in claim 20, where thinning results in a silicon epilayer thickness of less than about lOOnm.

22. A method as in claim 20, where etching yields an array of throughholes in the silicon epilayer comprising holes having a diameter of about 55nm and a center-to-center spacing of about 1 lOnm, and where a laser emission has a wavelength of about 1280nm.

23. A method as in claim 20, where etching yields an array of upstanding features having diameters of tens of nanometers.

24. A method as in claim 20, where applying a patterning mask applies a mask fabricated by a nano-scale self-assembly process.

25. A method as in claim 20, where applying a patterning mask applies a mask fabricated by an anodic aluminum oxide (AAO) fabrication process.

26. A method as in claim 19, further comprising smoothing a surface of the patterned layer to reduce optical losses due to scattering.

27. A method as in claim 19, further comprising post-processing the patterned layer to make structural modifications.

28. A method as in claim 19, further comprising fabricating electrical current injection means for injecting an electrical current into the patterned layer in a vertical injection configuration.

29. A method as in claim 19, further comprising fabricating electrical current injection means for injecting an electrical current into the patterned layer in a lateral injection configuration.

30. A method as in claim 19, where the patterned layer is formed over a substrate and is electrically pumped via at least one transistor device that is integrated on the same substrate.

31. A solid state optical emitter having an active region comprised of a layer comprised of an indirect-bandgap semiconductor material, said layer being patterned to have nano- scale features and emitting an optical signal characterized by an amplified spontaneous emission (ASE) spectrum, and further characterized by having an emission peak within the ASE spectrum that has a plurality of characteristics that are consistent with a laser emission.

32. A solid state optical emitter as in claim 31, where said ASE spectrum comprises wavelengths in the range of about 1240nm to about 1290nm, and where the emission peak is centered at about 1280nm.

33. A solid state optical emitter as in claim 31, where said layer is comprised of silicon and is patterned to exhibit an array of at least one of holes and pillars, and has a thickness of less than about lOOnm.

34. A solid state optical emitter device as in claim 31 operated at a cryogenic temperature.

35. A solid state optical emitter device comprised of an active region comprised of a layer of semiconductor material, said layer being patterned to comprise an array of features having a size, regularity and uniformity to cause said device to emit when pumped an optical signal having a plurality of characteristics that are consistent with a laser emission.

36. A solid state optical emitter device as in claim 35, where said plurality of characteristics that are consistent with a laser emission comprise threshold behavior, a linear power evolution, linewidth narrowing and a polarized emission.

37. A solid state optical emitter device as in claim 35, where said layer of semiconductor material is comprised of silicon, and where said optical signal has a wavelength of about 1280nm.

38. A solid state optical emitter device as in claim 35, where said layer is patterned to have at least one of an array of hole features and an array of upstanding features.

39. A solid state optical emitter device as in claim 35, where said layer of semiconductor material is comprised of silicon, and where said device is operated at a cryogenic temperature.

40. A solid state optical emitter device as in claim 35, where said layer of semiconductor material is comprised of indirect-bandgap semiconductor material, and where said optical signal is characterized by an amplified spontaneous emission (ASE) spectrum, and further characterized by having an emission peak within the ASE spectrum that has a plurality of characteristics that are consistent with the laser emission.