Patent Application
20090093074
s invention pertains generally to optical materials, and in particular to light-emitting, silicon-based nanocrystals, light emitting aperiodic photonic structures, and the fabrication thereof.
Conventional multilayer-deposition approaches have produced good quality periodic photonic structures, like Fabry-Perot Microcavities and distributed Bragg mirrors. More challenging is the fabrication of high quality aperiodic structures. An example of such a photonic structure is a deterministic aperiodic structure such as the one generate by a Thue-Morse sequence. This is just an example of aperiodic photonic structures and does not limit the approach we are proposing to this specific choice.
Specifically, an aperiodic structure generated by a Thue-Morse sequence is a structure obtained by the simple inflation rule .σT-M: A→AB, B→BA. Very recently the authors demonstrated omnidirectional reflectivity in a passive Thue-Morse photonic structure fabricated by a standard sputtering deposition technique. It is known theoretically that aperiodic structures show fractal distribution of sharp band-edge states with resonant transmission and strong field enhancement effect.
However, despite the big potential of aperiodic structures for enhancing light-matter interactions, no such aperiodic structures have been demonstrated combining both strong light-matter interaction and light emission. In fact, the standard materials approaches used to fabricate aperiodic photonic structures are not suited to obtain strong light emission homogeneously from all the layers constituting the structures. In other words, conventional materials used to fabricate aperiodic structures do not provide efficient light emission originating homogeneously from within the photonic structure itself.
Therefore, it is desirable to develop CMOS-compatible light emitting photonic structures that can produce efficient room temperature light emission homogeneously from the inside of photonic structure. Moreover, it is desirable to develop materials that can be used to fabricate aperiodic structures that obtain strong light-matter coupling homogeneously from all the layers constituting the structures.
Silicon (Si) has recently been shown to be a powerful material for integrated optics, modulation, switching, and even lasing. It has not, however, been proven to be an efficient light-emitting material. Light emission in bulk Si originates from a low-probability, phonon-mediated transition that unfavorably competes with fast, non-radiative recombination paths. The lack of efficient light emission in bulk Si has hampered the monolithic integration of electronic and optical devices on mass-produced Si-based chips.
Recent new techniques are providing methods to turn Si into a more efficient light-emitting material. New Si nanostructures have been synthesized that take advantage of quantum confinement to improve light-generation efficiency. Nevertheless, a need exists for further improvement.
A process is provided for improved light emission from silicon nanocrystals, a fundamental material system for CMOS-compatible light emitters. The disclosed methods may also be applied to other material systems that utilize a large number of light emitting centers of appropriate sizes. In particular, sequential thermal annealing enables the formation of a high density of silicon nanocrystals (Si-nc), favorable for better light emission and electrical injection, with CMOS-compatible matrices, e.g., Si, SiN, SiON, SiGe, etc.
In an aspect, the invention features a method of fabricating an aperiodic multilayer structure. The method includes depositing a thin film layer of SiO2 onto a substrate; depositing a thin film layer of SiNx upon the layer of SiO2 to form a structure; and thermally annealing the structure formed by depositing the thin film layer of SiO2 and the thin film layer of SiNx. The thermal annealing may be carried by an initial low temperature annealing process followed by a higher temperature annealing process.
In another aspect, the invention features a coupled aperiodic structure having fractal cavities. The coupled aperiodic structure may include a plurality of aperiodic multilayers, each aperiodic multilayer being constructed of thermally annealed thin film layers of SiO2 and SiNx; and a dielectric spacer positioned between each aperiodic multi layer.
In still another aspect, the invention includes a light emitting device. The light emitting device may include two aperiodic multilayer structures, each aperiodic multilayer structure being constructed of thermally annealed thin film layers of SiO2 and SiNx; and layers of rare earth atoms positioned between each aperiodic multilayer structure.
In another aspect, the invention includes a photonic quasicrystal planar device. The photonic quasicrystal planar device may include a channel waveguide structure with aperiodic etched trenches along a guiding direction, a core of the channel waveguide structure being constructed of thermally annealed SiNx; and a low index material formed within the aperiodic etched trenches of the channel waveguide structure.
In another aspect, an embodiment of the invention features a method for enhancing photoluminescence. The method includes providing a film over a substrate, where the film includes at least one of a semiconductor or a dielectric material. A first annealing step is performed at a first temperature in a processing chamber or annealing furnace. Thereafter, a second annealing step is performed at a second temperature in the processing chamber or annealing furnace. The second temperature is greater than the first temperature, and a second photoluminescence of the film after the second annealing step is greater than an initial photoluminescence of the film before the first annealing step.
One or more of the following features may be included. The substrate may remain in the processing chamber or annealing furnace between the first and second annealing steps. The substrate may be removed from the processing chamber or annealing furnace after the first step, and re-inserted into the processing chamber or furnace for the second step after a temperature of the processing chamber or annealing furnace is stabilized at the second temperature. The film may include silicon. The dielectric material may include or consist essentially of, e.g., SiO2, Si3N4, Si-rich silicon oxide, Si-rich silicon nitride, and/or Si-rich oxynitride. The first temperature may be selected from a range of 300° C. to 1300° C., preferably 400° C. to 1250° C., and more preferably 500° C. to 1200° C. The film thickness may be selected from a range of 0.1 μm to 5 μm.
The present invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. In the drawings, like reference numbering has been used throughout to designate identical or equivalent elements. It is also noted that the various drawings illustrating the present invention may not have been drawn to scale and that certain regions may have been purposely drawn disproportionately so that the features and concepts of the present invention could be properly illustrated. The drawings are only for purposes of illustrating a preferred embodiment or embodiments and are not to be construed as limiting the present invention, wherein:
a, 3b, and 5 are photoluminescence spectra of materials annealed in accordance with embodiments of the invention;
Features of the present invention are described in connection with preferred embodiments; however, there is no intent to limit the present invention to the embodiments described herein. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the present invention as defined by the appended claims.
Embodiments of the present invention may include light emitting optical devices characterized by aperiodic order and strong light-matter interaction. The application fields are integrated non-linear optics, low threshold optical amplifiers, on-chip optical sensors, optical switches, and all optical delay lines based on strong light dispersion in aperiodic photonic crystals structures.
More specifically, embodiments of the present invention may include deterministic aperiodic photonic materials (photonic quasicrystals) and structures, where critical and localized light states can trap light waves of several frequency simultaneously (multi-frequency photon localization), and the presence of narrow resonant states with high quality factors (high Q states), leads to dramatic enhancement effects in the electric field modes and to strong light dispersion effects with respect to regular photonic crystals structures.
Photonic quasicrystals are dielectric materials where the refractive index fluctuates without spatial periodicity, though the index profile is generated according to simple deterministic rules (such as the Fibonacci sequence: Fj+1=Fj+Fj−1). This class of perfectly ordered materials realizes an intermediate regime of geometrical organization between random structures and periodic ones.
Analogously to photonic band-gap materials, photonic quasicrystals can be realized by stacking together layers of different dielectric materials with thicknesses comparable to the wavelength of light. Moreover, photonic quasicrystals provide extremely complex fractal spectra, high Q resonance transmission states, localized and fractal photon states with sizeable field enhancement, and anomalous light diffusion. Lastly, photonic quasicrystals can effectively enable strong group velocity reduction (slow photons); strong light-matter interaction; light emission enhancement; gain enhancement; and/or nonlinear optical properties enhancement. As such, photonic quasicrystals can enhance the performance of light emitting devices, integrated non-linear optics, low threshold optical amplifiers, on chip optical sensors, optical switches, ultrashort optical pulse compression elements, and optical delay lines.
To fabricate the light emitting photonic, periodic photonic, and non-periodic photonic structures described here, a fabrication process utilizes thin film deposition of dielectrics followed by thermal annealing treatments that activates efficient room temperature light emission. In one embodiment of the present invention, the thin film dielectrics may be SiO2 and Si-rich nitride (SiNx).
The fabrication process may include deposition on transparent fused silica substrates through plasma enhanced chemical vapor deposition. However, several other thin-films fabrication procedures can be utilized.
In a specific example, silicon nitride layers are deposited using SiH4 and N2 as precursors while oxide layers are deposited using SiH4 and N2O. The substrate temperature during deposition is about 400° C. In order to maximize the effect of light scattering, the thickness dA,B of the two materials, SiNx (layer A) and SiO2 (layer B), has been chosen to satisfy the Bragg condition, dAnA=dBnB=λo/4, where nA (2.23) and nB (1.45) are the respective refractive indices and λo=1.65 μm.
Within a fully VLSI-CMOS compatible annealing window, the present invention utilizes a post-deposition annealing treatment in N2 atmosphere that produces active devices with efficient light emission from the layers of the photonic structures. In addition, the present invention yields little absorption losses in the visible range and intense broad band photoluminescence.
It is noted that the luminescence band can further be tuned by deposition of oxynitride (SiONx) thin films with variable stoichiometry.
Furthermore, it is noted that low temperature pre-annealing processes followed by higher temperature thermal annealing treatments in forming gas atmosphere can be utilized to control the spectral width of the emission band.
In one fabrication embodiment, various annealing treatments, ranging from 400° C. up to 1300° C. enable the fabrication of photonic structures that have a greater degree of flexibility and light emission control than structures produced by conventional fabrication processes. It is noted that the annealing time is determined according to the structure composition wherein the annealing time ranges from 1 minute to several hours. The thermal annealing post growth process activates the light emission homogeneously from the layers of the structures (interfaces).
Utilizing the fabrication process of the present invention, the aperiodic Thue-Morse photonic structures demonstrate interface quality comparable with conventionally fabricated periodic structures, allowing for the observation of complex transmission spectra with large field enhancement effects and light dispersion.
Sequential thermal annealing treatments are employed to improve the optical emission properties of Si-based materials, and to tune Si-cluster size and size distribution. As used herein, “sequential thermal annealing” refers to any combination of thermal annealing steps that includes low-temperature annealing and high-temperature annealing. “Low temperature” signifies any temperature lower than that of the main or primary annealing step.
Referring to
The film 100 may have a thickness selected from a range of, e.g., 0.1 μm to 5 μm (in one particular embodiment, the thickness is 1 μm).
The film 100 may be formed by, e.g., magnetron sputtering, plasma enhanced chemical vapor deposition (PECVD), low-pressure chemical vapor deposition (LPCVD), or other suitable techniques. For example, SiO2 may be formed by sputtering a silicon target with argon and oxygen. A silicon-rich oxide may be formed by sputtering Si and an SiO2 targets. A silicon-rich oxide may also be grown by, e.g., PECVD or LPCVD, or may be formed by implanting Si into a SiO2 film and annealing at a high temperature.
The substrate 110 may be a semiconductor substrate, including or consisting essentially of a group IV element or compound, such as Si, Ge, SiGe, and SiC; a III-V compound, such as GaAs, InGaAs, GaInP, GaN, InGaN, AlGaN, and InP; and/or a II-VI compound, such as CdTe and ZnSe. Examples of the semiconductor substrate include bulk Si and silicon-on-insulator (SOI).
Annealing steps following deposition cause formation of Si-nc inside the film. A high density of Si-nc, e.g., in the range of approximately 1015 to 1019/cm3, with an appropriate size, e.g., having a diameter in the range of about 1 to 10 nm, is highly preferred for good light emission from these material systems.
Referring to
In an embodiment of the invention, a first annealing step at a first temperature is performed in a processing chamber or annealing furnace. The first temperature may range from, e.g., 300° C. to 1300° C., preferably from 400° C. to 1250° C., and most preferably from 500° C. to 1200° C. The substrate and overlying film are subsequently subjected to a second annealing step in the same processing chamber or annealing furnace. The second annealing step is performed at a second temperature that is higher than the first temperature. The second temperature may range from, e.g., 300° C. to 1300° C., preferably from 400° C. to 1250° C., and most preferably from 500° C. to 1200° C. The photoluminescence of the film after the second annealing step is greater than the photoluminescence before the first annealing step.
The substrate may remain in the processing chamber or annealing furnace between the first and second annealing steps. Alternatively, the substrate may be removed from the processing chamber or annealing furnace after the first step, and re-inserted therein for the second step after the temperature of the processing chamber or annealing furnace is stabilized at the second temperature.
The effects of sequential thermal annealing steps on light emission were investigated, with the goal of increasing the density of Si-nc and to increase their emission intensity. Specifically, the role of sequential thermal annealing steps on the inducement of Si-nc nucleation and activation of efficient light emission was investigated in a controlled nitrogen atmosphere. After thermal annealing, strong near infrared (700-900 nm) light emission at room temperature under optical pumping was observed.
Room-temperature photoluminescence experiments were preformed by using a 488 nm Ar pump laser and a liquid nitrogen cooled InGaAs photomultiplier tube.
Low temperature pre-annealing treatment of reactively sputtered substoichiometric oxide (e.g., a SiOx matrix) films was performed to induce the formation of a large number of small Si clusters that can act as initial nucleation sites for a subsequent nucleation induced by a higher temperature treatment.
All of the experimental annealing treatments were performed in a controlled nitrogen atmosphere. Typical annealing temperatures ranged from 600° C. to 1200° C., and the total annealing time was kept fixed to 1 hour.
As shown in
b) illustrates the results of a more detailed investigation of the influence of the low temperature pre-annealing steps. Here, a pre-anneal was performed for 45 minutes at different temperatures between 600° C. and 1100° C., i.e., at 600° C. (curve 340), 800° C. (curve 350), and 1100° C. (curve 360), and no pre-anneal (curve 370), and a post-anneal was performed for 15 minutes at a fixed temperature of 1200° C. The annealing temperature profiles are illustrated in
Referring to
a and 6b illustrate the likely basis for the improvement of light emission by sequential annealing. Without sequential annealing (see
An example of the realized improvement in absorption loss by aperiodic Thue-Morse photonic structures fabricated by the concepts of the present invention is illustrated in
In another example of the fabrication process of the present invention, the deposited thin film dielectrics are thermal annealed, in order to activate broad band light emission from the Si-rich nitride layers. From this thermal annealing treatment, a broad emission band originating from the SiO2/SiNx interfaces can be realized, as illustrated in
The advantage of fabricating aperiodic photonic structures utilizing the concepts of the present invention is clearly shown in
More specifically, as illustrated in
Moreover, it is noted that the utilization of SiNx as a high refractive index and a broad band light emitting material enables effective transfer of the excitation to rare earth atoms (for instance erbium) through energy coupling mechanisms. The emission mechanism relies on the formation of nitrogen passivated small silicon clusters dispersed in the embedding Si3N4 dielectric host, in close analogy with Si/SiO2 superlattice systems. In other words, the inclusion of rare earth ions within the nitride or oxynitride structures fabricated by the processes of the present invention produces light emitting photonic structures characterized by efficient near infrared emission with a greater degree of structural flexibility.
On the other hand, the broad photoluminescence band in the multi-interface structures fabricated by the processes of the present invention provide an ideal candidate for efficient energy coupling effects with different rare earth atoms (Pd, Yb, etc) incorporated within the layers, and allows a wider emission tuneability range.
As noted above, the fabrication process of the present invention can be utilized to realize different aperiodic light emitting photonic structures schemes. The Thue-Morse generation rule is just one example of a deterministic prescription that generates a non-periodic layer sequence.
Another example, of photonic structure fabricated by the process of the present invention is illustrated in
It is noted that the energy transfer phenomena from Si-rich oxide and Si-rich nitride can be effective to enhance simultaneously the Er light emission. The combination of these light emitting materials with the strong mode localization effects occurring in photonic aperiodic dielectrics like Rudin-Shapiro and Thue-Morse structures lead to enhanced light-matter coupling effects leading to high excitation efficiencies for rare earth atoms.
It is noted that aperiodic and quasi-periodic luminescent photonic structures characterized by strong light-matter coupling and enhancements effects can be realized by the fabrication process of the present invention. As such, both linear (absorption, emission) and non-linear processes (second harmonic generation, third harmonic processes, light modulation) in the structures can be enhanced as a result of the strong electric field enhancement effects and density of optical modes modifications.
In particular, Thue-Morse and Rudin-Shapiro aperiodic photonic structures can be realized by layer deposition in order to achieve efficient light emission and eventually mirrorless light amplification (fractal laser) behaviour within an enhanced light-matter coupling regime.
It is further noted that light emission at 1.55 μm can be enhanced by Er doping in the SiOx and SiNx layers of the photonic structures. Deposition methods as Plasma Enhanced Chemical Vapor Deposition and RF magnetron sputtering can be utilized. In addition, based on the aperiodic photonic crystal approach described above, both passive (non-light emitting) and active (light emitting) structures can be realized. The passive device may consist of aperiodically arranged dielectric layers of Si, Si3N4, and SiO2 layers.
As noted above with respect to
It is noted that multi-frequency light emitting devices can be realized using the strongly localized light states in aperiodic multilayer structures (Thue-Morse and Rudin-Shapiro). These laser devices can operate at different frequencies corresponding to closely spaced, localized states. The underlying fractal behaviour of the transmission spectra can provide an integrated light source operating at multiple frequencies.
In a further embodiment, the fabrication process can be utilized to couple together sequentially these structures through a dielectric spacer, as illustrated in
It is noted that the strong mode coupling in these fractal coupled cavities enable the realization of THz signal modulators operating at the beating frequency of adjacent localized modes simultaneously excited by the internally generated light.
It is further noted that by embedding light emitting materials (Er, silicon nanocrystals) in the SiO2 core of a waveguide structures with photonic quasicrystal 1610 within cladding layers 1620, as illustrated in
It is noted that the present invention provides the possibility of demonstrating a light emitting device that profits from strong light-matter interaction in aperiodic material structures. The light emitting device can be realized by a proper choice of light emitting materials where light emission is activated through thermal annealing steps of the aperiodic structure. In particular, thermally annealed multilayers of SiO2 and SiNx, can be used or, additionally, infrared emitting rare earth atoms can be incorporated at specific locations inside the of SiO2 layers of the aperiodic structures.
It is also noted that optical delay lines can be realized by using the strong localized light field inside aperiodic structures based on the strongly suppressed group velocity at the localized modes. On the other hand, strong group velocity dispersion in aperiodic structures can be utilized to implement ultrafast optical pulse compression elements.
Another device that can be realized by the fabrication process of the present invention is a planar aperiodic waveguide as illustrated in
As noted above, a photonic quasicrystal distributed grating air trench planar waveguide realizes integrated multi-frequency filters and multi wavelength light emitting components. In addition, photonic quasicrystal distributed SiO2 trench grating realizes an integrated multi-frequency waveguide amplifier. The active medium (Er, silicon nanocrystals, ILL-V materials, dye molecules, etc.) is embedded inside the SiO2 trenches.
Lastly, polymers, liquid crystals, or other optically active materials can be used to fill the air trenches to vary the refractive index contrast of the structure realizing a tuneability of both the gap and the localized states that can be used for sensors and optical active devices integrated in a waveguide. This structure can be obtained by standard lithographic and etching CMOS processing and can be readily integrated on a planar silicon platform. The advantages of the planar aperiodically trenched waveguides rely in the huge light-matter interaction length with respect to vertical multilayers structures.
In summary, the fabrication process of the present invention realizes photonic quasicrystal devices that provide strong group velocity reduction (slow photons), strong light-matter interaction, light emission enhancement, gain enhancement, and/or nonlinear optical properties enhancement. Moreover, a viable and flexible solution for the realization of strong photonic enhancement and localization effects in high quality periodic and aperiodic photonic structures emitting at 1.55 μm is realized by the concepts of the present invention.
The fabrication process of the present invention is entirely compatible with CMOS processes; utilizes high index (refractive index ranging from 1.6 to 2.3) material to allow flexible design of high confinements photoriic devices with strong structural stability with respect to annealing treatments; realizes broad band light emission by allowing resonant coupling with rare earth atoms and other infrared emitting quantum dots; realizes better electrical conduction properties with respect to SiO2 systems; enables high transparency (low pumping and modal losses) in the visible range; and/or enables structural stability by allowing the realization of good quality light emitting complex photonic structures and photonic crystals structures.
The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative of the invention described herein. Various features and elements of the different embodiments can be used in different combinations and permutations, as will be apparent to those skilled in the art. The scope of the invention is thus indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced herein.
The present patent application is a continuation-in-part of and claims the benefit of and priority to both U.S. patent application Ser. No. 11/637,405, filed on Dec. 12, 2006, and U.S. patent application Ser. No. 11/113,624, filed on Apr. 25, 2005, which claims priority under 35 U.S.C. §119 from U.S. Provisional Patent Application Ser. No. 60/565,164, filed on Apr. 23, 2004; from U.S. Provisional Patent Application Ser. No. 60/564,900, filed on Apr. 23, 2004; and from U.S. Provisional Patent Application Ser. No. 60/631,041, filed on Nov. 24, 2004. The entire contents of all of these applications are hereby incorporated by reference in their entireties.
This invention was made with government support under the MIT Office of Sponsored Research Project Number 6894014, sponsored by the National Science Foundation, grant number DMR-0213282. The government has certain rights to this invention.
| Number | Date | Country | |
|---|---|---|---|
| 60565164 | Apr 2004 | US | |
| 60564900 | Apr 2004 | US | |
| 60631041 | Nov 2004 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11637405 | Dec 2006 | US |
| Child | 12137224 | US | |
| Parent | 11113624 | Apr 2005 | US |
| Child | 11637405 | US |
