WO2007067165A1

WO2007067165A1 - Enhanced electrical characteristics of light-emitting si-rich nitride films

Info

Publication number: WO2007067165A1
Application number: PCT/US2005/043762
Authority: WO
Inventors: Luca Dal Negro; Lionel C. Kimerling; Jurgen Michel; Jae Hyung Yi
Original assignee: Massachusetts Institute Of Technology
Priority date: 2005-12-05
Filing date: 2005-12-05
Publication date: 2007-06-14

Abstract

A device for electroluminescence, and a method for formation thereof, including a Si-rich silicon nitride film having a refractive index at 1.55 µm greater than or equal to about 2.1 and including a plurality of silicon nanoclusters having a density greater than or equal to 1017 cm-3, and an electron injector and a hole injector coupled to the Si-rich silicon nitride film so as to allow bipolar injection of electrons and holes into the Si-rich silicon nitride film.

Description

ENHANCED ELECTRICAL CHARACTERISTICS OF

LIGHT-EMITTING Si-RICH NITRIDE FILMS

Field of the Invention

[0001] The present invention relates generally to optical materials and particularly to light-emitting silicon-rich nitride films.

Background

[0002] 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 chips.

[0003] Several technological routes, however, are now open to turn Si into a more efficient light-emitting material. Recently, new Si nanostructures have been synthesized that take advantage of quantum confinement to improve light generation efficiency. High emission efficiencies up to 23% under optical excitation have been reported in porous Si after high pressure water vapor annealing and sizeable optical gain has been recently demonstrated in Si nanocrystals (Si-ncs) embedded in silicon dioxide (SiO₂) matrices, opening the race towards the fabrication of a fully Si-based laser. Moreover, it has been recently discovered that Si-ncs act as efficient energy sensitizers for rare-earth ions, particularly erbium (Er) ions, allowing broad band pumping of 1.55 micrometer (μm) light emission with almost 3 orders of magnitudes enhanced pumping efficiency. Nevertheless, porous Si and Si-nc embedded in SiO₂ matrices may not be suitable for the fabrication of reliable, optically efficient and stable electrically-driven light sources because of the insulating SiO₂ barriers and the slow exciton recombination lifetime [10- 100 microseconds (μs)].

Summary

[0004] The study and improvement of light emission from Si-rich (i.e., the content of Si is greater than the Si content in stoichiometric silicon nitride amorphous matrix - Si₃N₄) silicon nitride (SRN or SiN_x, wherein x is below the stoichiometric value of about 1.33) amorphous materials may have a significant impact on silicon microphotonics due to the combination of efficient light generation and more favorable electrical injection with respect to larger bandgap Si oxide-based materials. Visible, light-emitting electroluminescent devices with ~ 2% efficiency at room temperature and near-infrared emitting photonic structures have been achieved by following this approach.

[0005] CMOS-compatible approaches that can yield efficient light emission with fast

(i.e., on the order of nanoseconds), efficient and stable electrical excitation are needed. A possibility is the nucleation of Si nanoclusters in dielectric matrices with bandgaps smaller than that of SiO₂ and more favorable electrical properties. Following this approach, visible and near- infrared light-emitting Si clusters embedded in amorphous Si nitride matrices have been demonstrated. To provide optical amplification, a large density of small Si nanoclusters are provided in close proximity to rare earth ions to increase energy transfer rate. In an embodiment, processes and materials result in small Si nanoclusters strongly coupled with rare earth ions, after the rare earth ions are introduced into the matrix.

[0006] A method and structure are provided to enhance the electrical characteristics of CMOS-compatible light-emitting devices based on Si nanostructures in SRN. A device scheme enables efficient electrical injection and electroluminescence in silicon-based devices with broad-band near infrared and visible emission. Rare earth atoms, such as erbium (Er) ions, may be incorporated to achieve, e.g., 1.55 μm light emission within the device applications suggested here.

[0007] CMOS-compatible materials and processing are disclosed that enable the development of light-emitting devices that require low operation voltages and low processing temperatures. Low operation voltage has been achieved with post-deposition annealing steps and a method to make electroluminescence possible in SRN is proposed. The injection of electrons and holes into an active light-emitting layer of a bipolar device scheme is

demonstrated. A device structure emitting light at 1.55 μm is described.

[0008] In an aspect, the invention features a device for electroluminescence including a

Si-rich silicon nitride film having a refractive index at 1.55 μm greater than or equal to about 2.1 and including a plurality of silicon nanoclusters having a density greater than or equal to 10¹⁷ cm^"3. The term nanoclusters is herein used to indicate clusters having a size smaller than or equal to about 5 nm. The device also includes an electron injector and a hole injector coupled to the Si- rich silicon nitride film so as to allow bipolar injection of electrons and holes into the Si-rich silicon nitride film. [0009] One or more of the following features may be included. The refractive index of the Si-rich silicon nitride film may be greater than or equal to about 2.2, and/or lower than or equal to about 2.4. The density of the nanoclusters may be greater than or equal to about 10¹⁸ cm^"3, e.g., greater than or equal to about 10¹⁹ cm^"3. The density of the nanoclusters may be greater than or equal to about 10²⁰ cm^"3. An average diameter of the nanoclusters may be less than or equal to 2 nm. A thickness of the film may be selected from a range of about 10 nm to about 2000 nm, e.g., it may be greater than or equal to about 50 nm or 100 nm.

[0010] The electron injector may be disposed over a first side of the film, and the hole injector may be disposed over a second side of the film. The hole injector may include a conductive oxide having a p-type doping, e.g., indium tin oxide (ITO). The electron injector may include an n-type silicon substrate.

[0011] The Si-rich silicon nitride film may include a rare earth material. The rare earth material may be Er. An Er ion density in the Si-rich silicon nitride film may be selected from a range of about 10¹⁹ cm^"3 to about 10²¹ cm^"3.

[0012] At least a portion of the Si-rich silicon nitride film may define at least a portion of a waveguide, such as a core waveguide or a cladding waveguide. In an embodiment, the device may include a low index cladding layer disposed in contact with the Si-rich silicon nitride film and capable of confining light in the Si-rich silicon nitride core of the waveguide.

[0013] The device may be a component of an optical amplifier.

[0014] In another aspect, the invention features a method for manufacturing a device, the method including the steps of depositing by plasma enhanced deposition a Si-rich silicon nitride film over a substrate, the Si-rich silicon nitride film having a refractive index at 1.55 μm greater than or equal to about 2.1, wherein a temperature of the substrate during the deposition is greater than about 350⁰C. The method also includes thermally annealing the Si-rich silicon nitride film at a temperature selected from a range of 400 ⁰C to 1200 ⁰C, and coupling an electron injector and a hole injector to the Si-rich silicon nitride film so as to allow bipolar injection of electrons and holes into the Si-rich silicon nitride film.

[0015] One or more of the following features may be included. Plasma enhanced deposition may include plasma enhanced chemical vapor deposition. The annealing temperature may be selected from a range of 500 ⁰C to 900 ⁰C. An annealing duration is selected from a range of 1 minute to several hours, e.g., 5 hours, preferably to 30 minutes. For example, it may be greater than 2 minutes, or, in an embodiment, greater than or equal to about 5 minutes. Rare earth ions may be incorporated into the Si-rich silicon nitride film.

[0016] In another aspect, the invention features a method for manufacturing a device, the method including the steps of sputtering a Si-rich silicon nitride film over a substrate, the Si-rich silicon nitride film having a refractive index at 1.55 μm greater than or equal to about 2.1. The method also includes thermally annealing the Si-rich silicon nitride film at a temperature selected from a range of 400 ⁰C to 1200 ⁰C; and coupling an electron injector and a hole injector coupled to the Si-rich silicon nitride film so as to allow bipolar injection of electrons and holes into the Si-rich silicon nitride film.

[0017] One or more of the following features may be included. The annealing temperature may be selected from a range of 500 ⁰C to 900 ⁰C. An annealing duration is selected from a range of 1 minute to 5 hours. Rare earth ions may be incorporated into the Si- rich silicon nitride film.

[0018] In another aspect, the invention features a method for amplifying light, including the step of propagating an optical radiation having a wavelength selected from a range of about 1 micrometer to about 3 micrometers along a waveguide including a Si-rich silicon nitride material having a refractive index at 1.55 μm greater than or equal to about 2.1, and including silicon nanoclusters having a density greater than or equal to about 10¹⁷ cm^"3 and a rare earth material. The method also includes injecting holes and electrons into the Si-rich silicon nitride material so that energy transfer occurs between nanoclusters and erbium ions, thereby amplifying the optical radiation.

Brief Description of Figures

[0019] Figures Ia and Ib are, respectively, a graph of I- V characteristics illustrating an increase in current density due to post-deposition annealing, and a schematic diagram of a device structure that can be used for making such measurements;

[0020] Figures 2-9 are schematic cross-sectional representations of devices formed in accordance with aspects of the invention;

[0021] Figure 10 is a graph of the I-V characteristics of SRN showing current density dependence on post-deposition annealing temperature; [0022] Figure 11 a and 11 b are graphs of I-V characteristics of devices having an ITO transparent electrode and formed, respectively, on different types of substrates;

[0023] Figure 12a and 12b are, respectively, a graph of electroluminescence from the device of an aspect of the invention, overlapped with a graph of photoluminescence (PL) for comparison, and a schematic diagram of an exemplary device structure for achieving electroluminescence;

[0024] Figures 13a and 13b are schematic diagrams illustrating energy transfer in Si-nc embedded Er: SRN and Er atomic levels;

[0025] Figure 14 is a graph of a PL spectra showing Er:SRN emission from resonant and non-resonant pumping in the 1.55 um range;

[0026] Figures 15 is a graph of a PL spectra showing Er: SRN emission from resonant and non-resonant pumping in the 1.55 μm range;

[0027] Figure 16a, 16b, 16c, and 16d are graphs illustrating the PL of SRN and SRO materials as a function of refractive index and annealing temperature; Figure 16e is a graph illustrating the integrated PL intensity of SRN material versus annealing time at an annealing temperature of 700 ⁰C;

[0028] Figures 17a, 17b, 17c, and 17d are graphs illustrating results of micro-Raman and

TEM analysis of SRN and SRO samples;

[0029] Figures 18a and 18b are graphs illustrating a comparison of emission efficiencies of SRN and SRO samples;

[0030] Figures 19a and 19b are graphs illustrating the emission and transmission spectra of an SRN sample;

[0031] Figure 20 is a graph illustrating PL lifetime in an SRN sample at different wavelengths; and

[0032] Figure 21 is a graph illustrating is a graph illustrating the effect of temperature on

PL intensity in an SRN sample.

[0033] Like-referenced features represent common features in corresponding drawings.

Detailed Description

[0034] SRN materials offer the advantages of efficient photoluminescence, fast recombination time, materials reliability, and strong energy sensitization of rare earth atoms (particularly Er) because of the better electrical characteristics in comparison to Si-rich oxide (SRO). Nitride materials may be doped with Er and other rare earths (Yb, Nd, Pr, Tm, Ho, etc.) to extend the emission range in the near infrared region. Sputtering, plasma enhanced chemical vapor deposition (PECVD) or similar growth technique may be utilized to fabricate the materials. A post-deposition annealing process following the deposition is performed to induce or enhance the formation of Si nanoclusters in the matrix. These nanoclusters may be crystalline or amorphous. They may be hydrogen terminated. Electrical properties strongly depend on the post-deposition annealing conditions and may, thereby, be controlled.

[0035] Referring to Figure Ia, thermal annealing also leads to a dramatic increase, i.e., more than two orders of magnitude, in the current density of the SRN materials prepared in accordance with aspects of the present invention in comparison to similar materials fabricated without a thermal annealing step. Referring to Figure Ib, the device structure used to demonstrate the effect shown in Figure Ia is a unipolar device 100 including a p+ Si substrate 110, a SiN_x layer 120 (which may respectively be prepared in accordance with an embodiment of the present invention or, for comparative purpose, without the post-annealing step), and first and second gold contact layers 130, 140 disposed ih contact with the SiN_x layer 120 and the p+ Si substrate 110, respectively. The SiN_x layer 120 of Figures Ia and Ib has a thickness of about 700 nm and has been deposited by PECVD (see below) on the p+ Si substrate 110. The post- depostion annealed SRN sample of Figure Ia has been annealed at 700⁰C for 10 minutes.

[0036] A device structure, e.g., including a single mode ridge waveguide operational at 1.55 μm, with enhanced characteristics, may be manufactured as follows. Referring to Figure 2, a bottom cladding layer 200 including, e.g., silicon dioxide (SiO₂) or silicon oxynitride (SiON) with low N content, is formed over a semiconductor substrate 210 that includes or consists essentially of silicon, e.g., a transparent fused silica substrate. It is to be understood that, in case the substrate 210 is made of silica, the bottom cladding layer 200 may be omitted. The bottom cladding layer 200 may be deposited or grown, and may have a thickness tj of, e .g., 3 μm or higher.

[0037] An SRN film 220 is formed over the substrate 210, e.g., over the bottom cladding layer 200. Intermediate layers (not shown) may be interposed between the cladding layer 200 and the SRN film 220. SRN film 220 has a thickness t₂ selected in the range of, e.g., 0.01 - 2 μm. SRN film 220 may have a refractive index at 1.55 μm of greater than or equal to 2.1, e.g., greater than or equal to about 2.2. In some embodiments, the refractive index of the SRN film is lower than or equal to about 2.4. This refractive index that is not less than 2.1 is the refractive index of solely the SRN material (i.e., of solely the content of Si and N), without taking into consideration any dopants or other materials possibly included in the layer 220. The refractive index is believed to be a direct measure of extra silicon (with respect to the stoichiometric level) in the SRN material. A higher index indicates a higher concentration of Si. In such materials, even at low temperatures, the Si will precipitate into very small clusters.

[0038] The SRN film 220 may be deposited by PECVD using, e.g., an Applied Materials

Centura DxZ chamber with precursors of, e.g., silane (SiH₄) and nitrogen (N₂). A substrate temperature during deposition may be kept greater than about 350⁰C, e.g., about 400 ⁰C.

Additional processing parameters may include a nitrogen plasma having a power ranging from 100 watts (W) to 600 W, preferably 400 W to 500 W. The pressure during the deposition may range between 1 to 6 Torr, preferably 3 to 5 Torr, and the N₂ flow rate may be in the range of 1000 to 10000 seem, preferably 3000 to 5000 seem while the SiH₄ flow rate may range between 100 and 1000 seem, preferably 300 to 500 seem, depending on the film stoichiometry that is desired. For example, in a particular embodiment, the following parameters may be used to form SRN having a refractive index of 2.26: nitrogen plasma power of 440 W, pressure of 3 Torr, temperature of 400 ⁰C, N₂ flow rate of 4900 seem, and SiH₄ flow rate of 140 seem.

[0039] A rare earth material, such as Er, may be incorporated into the SRN film 220 during PE-CVD. Alternatively, Er ions may be implanted into the SRN film in a subsequent process step. The presence of rare earth ions in close proximity to a relatively high density of small Si nanoclusters may improve the electroluminescence of the formed device by increasing the energy transfer rate, as discussed below. A density of a rare earth ion, e.g., Er, in the Si-rich silicon nitride film may be selected from a range of about 10¹⁹ cm^"3 to about 10²¹ cm^"3

[0040] Alternatively, SRN film 220 may be formed by sputtering. The SRN material may be directly sputtered from e.g., Si and Si3N₄ targets. SRN films containing a rare earth, e.g., Er-doped SRN films (Er: SRN), may be fabricated by direct magnetron co-sputtering from Er, Si, and Si₃N₄ targets. An argon gas flow rate may range between 1 to 100 seem, preferably 20 seem, the RF plasma power on the Si target may range between 10 to 500 W, on the Er target from 10 to 200 W, and on the silicon nitride target from 10 to 500 W, depending on the film stoichiometry that is desired. The temperature of the substrate may be uncontrolled, e.g., it may be kept at room temperature. In a particular embodiment, the argon gas flow rate may be 20 seem, the RF plasma power on the Si target may be 300 W, on the Er target may be 20 W, and on the silicon nitride target may be 300 W.

[0041] To induce or enhance the nucleation and growth of Si nanoclusters, the SRN film 220 may be annealed at a temperature selected from a range of 400 ⁰C to 1200 ⁰C, preferably from a range of 500 ⁰C to 900 ⁰C, most preferably from a range of 600 ⁰C to 800 °C. The annealing may be performed in a N₂ rich atmosphere, and a duration of the annealing may be selected from a range of 1 minute to several hours, e.g., 5 hours. Preferably the annealing duration is greater than or equal to 2 minutes, more preferably greater than or equal to 5 minutes. Preferably it is shorter than or equal to 30 minutes, e.g., shorter than or equal to 10 minutes.

[0042] The density of the silicon nanoclusters may be greater than or equal to 10¹⁷ cm^"3, preferably greater than or equal to about 10¹⁸ cm^"3, more preferably greater than or equal to about 10¹⁹ cm^'3, and even more preferably greater than or equal to about 10²⁰ cm^"3. The average diameter of the nanoclusters may be less than or equal to about 2 nm. [0043] Referring to Figure 3, SRN film 220 is patterned to define a channel 300. The patterning may be performed by, e.g., a photolithographic process followed by removal of portion of the SRN film 220 by a selective dry etch. The channel 300 may have a width Wj of, e.g., 700 nm and a height h_; equal to the thickness t₂ of the SRN film 220, e.g., 500 nm.

[0044] Referring to Figure 4, a first contact material 400 is deposited over channel 300 and exposed bottom cladding layer 200 portions. The first contact material may be a conductive and transparent material suitable for use as both a cladding layer and an electrode, such as ITO, indium zinc oxide (IZO), transparent SiC, tin oxide (SnO), or polycrystalline Si, and may have a thickness t₃ that provides for low loss at 1.55 μm , e.g., 100 - 400 nm. In an embodiment, the first contact material 400 is a p-type material, e.g., any of the listed materials processed to behave as a hole injector. For example, polycrystalline Si may be doped by an implantation of boron ions to be p-type. In another embodiment, the first contact material 400 is an n-type material, e.g., any of the listed materials processed to behave as an electron injector. For example, polycrystalline Si may be doped by an implantation of arsenic ions to be n-type. The first contact material 400 may be deposited by, e.g., sputtering or PEVCD. [0045] Referring to Figure 5, first contact material 400 is selectively removed such that a portion of the first contact material remains on a first side 500 of the channel 300, thereby forming a first electrode 510. The first contact material 400 may be selectively removed by, e.g., dry etching, with an etch chemistry designed to protect the portions of the contact material disposed on the sidewalls of the channel.

[0046] Referring to Figure 6, an overclad layer 600 is conformally deposited over the channel 300 and over the first electrode 510. The overclad layer 600 may be a dielectric layer such as an oxide, e.g., SiO₂, having a thickness t₄ of, e.g., 200 nm. A photoresist layer (not shown) is deposited over the overclad layer 600 and patterned. Subsequently, an opening 610 is defined in the overclad layer 600 by, e.g., a dry etch, to expose a second side 620 of the channel 300. A second contact material 630 is deposited over the overclad layer 600 and on the second side 620 of the channel 300 to define a second electrode 640 (see Figure 7). The second contact material 630 may be a conductive material and transparent material, such as one of the materials that may be used for the first contact material, i.e., ITO, IZO, transparent SiC, SnO, or doped polycrystalline Si, and may have a thickness t₅ of, e.g., 50 - 200 nm. The materials for the first and second contact materials 400, 630 may be selected so that they are capable of injecting holes and electrons, respectively, into the channel waveguide 300. Thus, if the first contact material is treated to be a hole injector, i.e., p-type, the second contact material may be treated to be an electron injector, i.e., n-type. The material selected for the first and second contact materials 400, 630 typically has a low resistivity, e.g., <10^"2 Ohm cm.

[0047] Referring to Figure 7, portions of the second contact material 630 disposed on the overclad layer 600 are removed by, e.g., a wet etch, while second electrode 640 remains. In an embodiment, the first electrode 510 is an electron injector, and the second electrode 640 is a hole injector.- A device 700 may be a component of an optical amplifier and may include (i) a SRN film defining channel 300, i.e., a waveguide core, with the SRN film having a refractive index at 1.55 μm greater than or equal to about 2.1 and including a plurality of Si nanoclusters having a density greater than or equal to 10¹⁷ cm^"3, and wherein (ii) an electron injector and a hole injector are coupled to the SRN material of the channel 300, thereby allowing bipolar injection of electrons and holes into the SRN film. Device 700 also includes a low index cladding material disposed in contact with the Si-rich silicon nitride film and capable of confining light in the Si- rich silicon nitride core of the waveguide. [0048] In an alternative embodiment, device 700 has another configuration, and is formed as follows. Referring to Figure 8, after the (optional) formation of bottom cladding layer 200 over substrate 210, a first conductive layer 800, e.g., an n-type material, e.g., n+ polysilicon layer, is formed over the bottom cladding layer 200. The first conductive layer 800 may have a thickness tβ of, e.g., 50 ran - 200 nm. Thickness tø is selected such that resistivity is about 10^"2 Ohm cm or less; the resistivity may be higher if optical losses are unacceptably high. The SRN film 220 according to an aspect of the present invention is deposited over the first conductive layer 800. A second conductive layer 810, e.g., ITO, IZO, SnO, transparent SiC, or p-type polycrystalline silicon, is formed over the SRN film 220. The second conductive layer 810 may have a thickness t₇ of, e.g., 50 - 200 nm. Thickness tβ is selected such that resistivity is about 10^" ² Ohm cm or less; the resistivity may be higher if optical losses are unacceptably high. A photoresist layer (not shown) is deposited over the second conductive layer 810 and patterned.

[0049] Referring to Figure 9, portions of the first conductive layer 800, SRN film 220, and second conductive layer 810 are selectively removed by, e.g., a dry etch. Thus, channel 300 is defined, disposed between two lateral electrodes, i.e., first electrode 510 defined by a portion of the first conductive layer 800 and second electrode 640 defined by a portion of the second conductive layer 810. Overclad layer 600 is conformally deposited over the channel 300 and second electrode 640. In an embodiment, first electrode 510 is an electron injector, e.g., an n- type silicon substrate or an n-type conductive layer, disposed over a first side of the SRN film 220 and second electrode 640 is a hole injector, e.g., a conductive oxide such as ITO, disposed over a second side of the SRN film 220, with both electrodes coupled to the SRN film to allow bipolar injection of electrons and holes into the SRN film.

[0050] The layer structures illustrated above may be used to amplify light by propagating an optical radiation having a wavelength selected from a range of about 1 micrometer to about 3 micrometers along a waveguide comprising (either in the core portion, as shown above, or in the cladding portion) a Si-rich silicon nitride material having a refractive index at 1.55 μm greater than or equal to about 2.1, and comprising silicon nanoclusters having a density greater than or equal to about 10^π cm^"3 and a rare earth material, and injecting holes and electrons into the Si- rich silicon nitride material so that energy transfer occurs between nanoclusters and erbium ions, thereby amplifying the optical radiation. Experimental results illustrating effects of thermal annealing and rare earth incorporation

[0051] SRN samples were prepared as discussed above with reference to Figure 2, both by PE-CVD and sputtering. Comparative SRO samples were deposited by a reactive RF magnetron in an O₂/Ar atmosphere using a Kurt J. Lesker Co. CMS 18 sputtering system. [0052] Room temperature photoluminescence (PL) experiments were performed using a

488 nm Ar pump laser and a liquid nitrogen cooled InGaAs photomultiplier tube. PL decay measurements were obtained using the 457 nm line of a nitrogen laser pumped dye laser (pulse width 1 ns, 10 Hz repetition rate).

[0053] As indicated above with reference to the discussion regarding the formation of the device 700, preferred post-deposition annealing treatments may be performed at temperatures in the range between 300 ⁰C - 1200 ⁰C and the annealing time may be varied from 1 minute to several hours, e.g., 5 hours. In Figure 1, only representative data are included for a proof of concept demonstration. For better understanding and utilization of the innovative method, several post-deposition annealing experiments were perfoπned with the same strategy by changing post-deposition annealing temperature.

[0054] Referring to Figure 10, one can see that current density 1000 in a device as shown in Figure Ib comprising a 200 nm-thick SiN_x film increases with increasing annealing temperature. The annealing duration for the data in Figure 10 was 10 minutes. Without the annealing step, the current density 1010 through the device is negligible. This is a direct demonstration of the enhanced electrical characteristics of light-emitting SRN films due to post- deposition annealing.

[0055] It is believed that post-deposition annealing treatments not only induce or enhance the formation of light-emitting Si clusters but may also improve the electrical injection properties of SRN films by favoring both carrier( e.g., electron or hole) tunneling from the matrix and percolation (hopping) transport. The improved electrical behavior (e.g., as shown in Figure Ia) of thermally annealed SRN films according to an aspect of the present invention may be explained by hopping conduction through percolation clusters of empty localized states that can be associated both to the amorphous SRN matrix and to the Si-nanocluster interfaces. However, the dramatic effect of the post-deposition annealing treatment on the electrical transport of SRN films suggests the onset of a conduction percolation threshold associated with the high density of Si clusters obtained in accordance with an aspect of the present invention (e.g., by the combination of a suitable Si-content and a suitable annealing temperature and, possibly, a suitable substrate temperature).

[0056] The effect of thermal annealing on the electrical conduction properties of SRN, as discussed above, has been demonstrated, and p-i-n light-emitting structures have been fabricated that demonstrate the possibility of achieving electroluminescence by bipolar injection. Referring to Figure 12b, an exemplary structure used to demonstrate this behavior is a p-i-n device structure 1200 that includes an ITO electrode 1210 disposed over an SRN film 1220. The SRN film is formed over an n-H--Si substrate 1230 as described above (post-deposition annealing temperature 900 ⁰C), and a gold electrode 1240 is disposed on a backside of the substrate 1230. In the illustrated device 1200, the ITO electrode 1210 has a thickness of 100 nm, the SRN film has a thickness of 700 nm, and the gold electrode 1240 has a thickness of 100 nm. With this structure, diode behavior was clearly achieved, as evidenced by I-V measurements shown in Figure l ib.

[0057] Figure l la shows that no rectifying I-V characteristics were obtained with a comparative device similar to that shown in Figure 12b but having a p-i-p doping profile obtained by substituting the n++-Si substrate 1230 with a p-type Si substrate. Using p-i-p structures, it was demonstrated that only unipolar carrier injection is possible (ITO here has p- type doping and acts as a hole injector).

[0058] In Figure 12a, an electroluminescence (EL) curve of the device shown in Figure

12b is overlapped with a photoluminescence (PL) curve of the same device for direct comparison.

[0059] As mentioned above, Er or other rare earth atoms may be incorporated into the above described SRN materials and/or devices to extend the emission range to infrared wavelengths. The efficient energy transfer between silicon nanoclusters and Er ions profits from both the advantages of quantum size effects in Si and rare earth doping, leading towards the integration of CMOS technology with 1.54 μm light sources.

[0060] In particular, Er-doped SRO systems (Er: SRO) have been extensively

investigated and also, efficient energy sensitization has been demonstrated for low temperature annealed (600 - 800 ⁰C) Er: SRO samples produced by magnetron co-sputtering. The typical mechanism used to explain energy transfer between Si-ncs and Er ions is Fδrster-Dexter non- radiative energy coupling with a transfer rate that is directly proportional to the donor (SRO) emission rate. However, the Applicant believes that the validity of this simple model is questionable since only inefficient light emission (no appreciable SRO emission rate) may be obtained in the case of low temperature annealed samples (donors) without the presence of Er atoms (acceptor). It is possible that the efficient energy transfer observed for low temperature annealed EnSRO materials may be assisted by non-radiative defect states within the SRO matrix bandgap.

[0061] Electroluminescence of Er-doped SRN devices may be achieved within the same device structures proposed above, e.g., the waveguide 700 illustrated in Figures 7 and 9. Energy from Si-nanoclusters may be transferred to Er atoms inside the matrix and emission may thus be generated from Er atoms with a low operation voltage.

[0062] Referring to Figures 13a and 13b, the behavior of the devices described above may be clarified by taking into consideration the optical pumping mechanism. If an Er atom is to be pumped optically, a pumping wavelength should be resonant with atomic levels.

Otherwise, Er cannot be excited. That is, resonant pumping 1300 can excite Er atom directly, but non-resonant pumping 1310 cannot excite Er atom directly (Figure 13b).

[0063] However, if energy transfer involves Si-nanocrystals, Er can be excited.

Referring to Figure 13a, Si-nanocrystal mediated energy transfer from Si-nanocrystals in SRN to Er atom is illustrated. First, a Si-nanocrystal is excited through optical pumping and energy is transferred to Er atom. Then, Er atom is de-excited and emits light, which may be used to amplify an optical radiation.

[0064] ' Referring to Figure 14, energy transfer from Si-nanocluster to Er atoms is illustrated. In contrast to direct optical pumping, Er atoms may be excited even through non- resonant pumping 1400. With both non-resonant and resonant pumping, light emission from Er atoms is attained through energy transfer phenomena.

[0065] Utilization of energy transfer phenomena in SRN-based light emitting dielectrics may be particularly useful for the realization of light amplifiers on a Si chip. In particular, an electrically or optically driven light-amplifier based on an Er: SRN channel/ridge waveguide structure with light-emitting core and transparent cladding electrodes may be fabricated.

Furthermore, the illustrated bipolar-injection schemes in thermally annealed SRN matrices enable the achievement of CMOS-compatible Er-doped efficient devices operating at low voltage.

[0066] The possibility of extending the emission wavelength range of SRN optical devices to the strategically relevant 1.55 μm region has been experimentally demonstrated, based on efficient energy transfer to erbium ions. Referring to Figures 14 and 15, one can clearly see that 1.55 μm Er-related emission may be efficiently excited by pumping out of the Er transitions resonance, using a pumping wavelength (457 nm) where Er ions are non-absorbing (Figure 14). The almost totally overlapping Er emission spectra obtained under resonant (488 nm) and non- resonant (457 nm) pumping conditions, directly show that Er excitation in Er-doped SRN is substantially mediated by the SRN matrix that efficiently transfers excitation energy to the Er ions, as schematically shown in the inset of Figure 15. The lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) are indicated.

[0067] Referring to Figures 16a - 16d, the effect of film stoichiometry (as measured through the sample refractive index variations associated with the excess Si content) and post deposition annealing temperature on the emission intensity of SRN and SRO samples has been summarized. The SRN samples were obtained by the PE-CVD technique described above. In Figures 16a and 16b, trends of the integrated PL intensity versus the SRN and SRO film refractive indices, respectively, as measured at 1.55 μm by a prism coupling technique are illustrated. The SRN samples of Figure 16a have been post-deposition annealed at 700⁰C for 10 minutes, while the SRO samples of Figure 16b have been post-deposition annealed at 1100⁰C for about 1 hour. Figures 16c and 16d illustrate the integrated PL intensity versus the post-deposition annealing temperatures for the same materials. The SRN samples of Figure 16c have a refractive index of about 2.2 and have been post-deposition annealed for 10 minutes, while the SRO samples of Figure 16d have a refractive index of about 1.7 and have been post-deposition annealed for about 1 hour. Referring to Figure 16e, the integrated PL intensity for SRN samples versus annealing time is illustrated. The annealing temperature was 700 ⁰C. The result of the study shows that the better light emission performances are obtained for SRN samples with higher Si content (refractive index n=2.23), after thermal annealing at 700⁰C for a duration in a range of about 10 minutes to 1 hour. For comparison, Figures 16b and 16d show the light emission optimization trends versus film refractive index and annealing temperature (fixed time at 1 hour) for SRO films deposited by magnetron sputtering. In this case, the integral PL intensity is higher for films with a refractive index of 1.7 annealed at 1150⁰C for 1 hour, as a result of a competitive interplay between the nucleation of luminescent Si clusters with different sizes, emission efficiencies, and cluster density. However, the data indicates that a major difference exists between SRN and SRO light-emitting systems. In fact, the annealing time (1 hour) and temperature (1150⁰C) for increasing the Si-nanocluster light emission in oxide systems are typically much greater than what may be required to activate efficient light emission in SRN systems. This suggests that the growth kinetics in nitride films favor the formation of smaller Si clusters at a faster rate and for lower supersaturation than in the case of SRO films.

[0068] To confirm this assumption, micro-Raman and TEM analyses were performed on the best emitting SRN and SRO samples. Figure 17a shows the micro-Raman spectra of a reference bulk silicon (c-Si) sample (dash-dot), an as-deposited (not annealed) SRN sample produced by direct magnetron sputtering (dashed line) and a thermally annealed (700⁰C, 10 minutes) PECVD (n=2.2) deposited sample (solid line). For sputtered SRN samples with no substrate heating, only a broad Raman band due to an amorphous Si network is observed (dashed line). In contrast, in the case of thermally annealed SRN samples deposited by PE-CVD, a broadened, asymmetric and shifted Raman peak with respect to bulk Si can be clearly observed (solid line). Additionally, for the PECVD sample, the two-phonon optical (~ 900 cm^"1) and acoustical (~ 300 cm^"1) scattering bands are strongly enhanced with respect to both the bulk Si and the sputtered SRN samples. The presence of a significantly shifted (Δv ~ 15 cm^"1) and asymmetrized one-phonon Raman peak for PECVD deposited SRN is direct evidence of the formation of small Si clusters embedded in the amorphous silicon nitride matrix. The physical origin of the broadened Si peak is related to the uncertainty in the Si-cluster phonon momentum q that allows modes with q≠ 0 to contribute in the Raman spectrum. This general physical picture, referred to as phonon bottleneck, may be quantitatively described within a

phenomenological model that accounts for the lineshape of the TO one-phonon modes of quantum confined Si clusters.

[0069] Referring to Figure 17b, the simulated one-phonon Raman lineshape

corresponding to Si clusters with decreasing size is shown. The one-phonon Raman lineshapes may be obtained by calculating the integral transform:

where L is the average Si-nc radius, Fo is the linewidth of the longitudinal optical (LO) bulk Si phonon, a is the Si lattice constant, and the phonon dispersion of the bulk material as given by the relation ω²fø) = A +B cos (πq/2) with A =1.714 x 10⁵ cm^"2 and £ = 1.000 x 10⁵ cm^"2. This phenomenological approach allows one to simulate the experimental Raman data and to estimate an average size for the quantum confined scattering particles. This procedure was applied to SRO samples (annealed at HOO⁰C for 1 hour) and SRN samples (annealed at 700 ⁰C for 10 minutes), and consistency with the results of TEM analysis (Figures 17c and 17d) was checked. In the case of SRO samples, an average radius of ~ 3 nm was estimated, in good agreement with the TEM image (Figure 17c). In the case of thermally annealed SRN, the micro-Raman data are compatible with the presence of smaller Si clusters with an estimated size less than 2 nm, as fitted within the phonon confinement Raman model (see Figure 17d). In addition, by TEM analysis (Figure 17d), the presence of small silicon clusters in thermally annealed SRN was demonstrated, whose average size is estimated to be ~ 1 to 2 nm.

[0070] To quantitatively compare the emission efficiency of SRN versus SRO light- emitting systems, the external PL quantum efficiency (PLQE) of the better emitting PECVD SRN film (n=2.2, 700 ⁰C, 10 minutes) and SRO samples (n=1.7, 1150 ⁰C, 1 hour) were measured. Figure 18a shows a comparison between the normalized room temperature emission spectra of SRO (solid line) and SRN (dashed line) samples. The emission spectra of the two samples are similar and consist of broad emission bands centered around 900 nm. A PLQE of 7% was measured for the SRN samples, following the procedure described in T.-W.F. Chang et al., Synthetic Metals 148, 257 (2005) and de Mello et al., Adv. Mat. 9, 230 (1997), incorporated herein by reference in their entirety. In the case of the optimized SRO reference samples, a PLQE of 4.5% was obtained. The histograms of the corresponding integrated PL intensities are shown in Figure 18b. Referring to Figures 19a and 19b, the 7% efficient near-infrared SRN emission spectrum was directly compared to the optical transmission spectrum. It is clear that the emission band (Figure 19a) is strongly Stokes-shifted with respect to the onset of the measured absorption edge (Figure 19b), suggesting the strongly localized nature of the emitting centers.

[0071] First principles theoretical calculations indicate that light emission in SRN samples in embodiments of the present invention may originate from strongly localized, nitrogen-related exciton states at the surface of small (~ 1 to 2 nm) Si clusters embedded in an amorphous Si₃N₄ network. Figure 20 shows the measured wavelength dispersion of the PL lifetime of the best emitting SRN sample. The SRN lifetime is described by a double exponential function with a resolution limited sub-nanosecond fast decay component and a longer decay component that ranges between 1 and 5 ns, depending on the observation wavelength. The inset shows two representative PL decay traces. The temperature behavior of the SRN light emission is shown in Figure 21. The SRN emission shows negligible temperature quenching (approximately a factor of 4) over a wide temperature range from 4 K up to 330 K. In addition, no appreciable emission linesliape modifications have been observed (Figure 21).

[0072] The temperature-dependent PL data can be fit using a simple phenomenological model based on the thermal ionization of localized carriers from a radiative nitrogen defect state. However, the microscopic nature of the radiative nitrogen defects suggested in Deshpande et al., J. Appl Phys. 77, 6534 (1995), incorporated herein in its entirey, may be associated with the strongly localized energy state introduced by surface nitrogen bridging configurations within the HOMO-LUMO gap of small Si clusters embedded in the nitride matrix. According to this interpretation, the fast sub-nanosecond PL decay component is associated with a non-radiative exciton trapping time on the nitrogen sites while the longer (ns) decay results from the localized exciton recombination time.

[0073] 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 rather than limiting on the invention described herein. 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 intended to be embraced therein.

Claims

[0074] What is claimed is:

1. A device for electroluminescence comprising:

a Si-rich silicon nitride film having a refractive index at 1.55 μm greater than or equal to about 2.1 and comprising a plurality of silicon nanoclusters having a density greater than or equal to 10¹⁷ cm^"3; and

an electron injector and a hole injector coupled to the Si-rich silicon nitride film so as to allow bipolar injection of electrons and holes into the Si-rich silicon nitride film.

2. The device of claim 1, wherein the refractive index of the Si-rich silicon nitride film is greater than or equal to about 2.2.

3. The device of claim 1, wherein the refractive index of the Si-rich silicon nitride film is refractive index lower than or equal to about 2.4.

4. The device of claim 1, wherein the density of the nanoclusters is greater than or equal to about 10¹⁸ cm^"3.

5. The device of claim 1, wherein the density of the nanoclusters is greater than or equal to about 10¹⁹ cm^"3.

6. The device of claim 1, wherein the density of the nanoclusters is greater than or equal to about 10²⁰ cm^'3.

7. The device of claim 1, wherein an average diameter of the nanoclusters is less than or equal to 2 nm.

8. The device of claim 1, wherein a thickness of the film is selected from a range of about 50 nm to about 2000 nm.

9. The device of claim 1, wherein the electron injector is disposed over a first side of the film; and the hole injector is disposed over a second side of the film.

10. The device of claim 9, wherein the hole injector comprises a conductive oxide having a p-type doping.

11. The device of claim 10, wherein the conductive oxide comprises ITO.

12. The device of claim 9, wherein the electron injector comprises an n-type silicon substrate.

13. The device of claim 1, wherein the Si-rich silicon nitride film comprises a rare earth material.

14. The device of claim 13, wherein the rare earth material is Er.

15. The device of claim 14, wherein an Er ion density in the Si-rich silicon nitride film is selected from a range of about 10¹⁹ cm^"3 to about 10²¹ cm^"3.

16. The device of claim 1, wherein at least a portion of the Si-rich silicon nitride film defines a waveguide core, the device further comprising: a low index cladding layer disposed in contact with the Si-rich silicon nitride film and capable of confining light in the Si-rich silicon nitride core of the waveguide.

17. An optical amplifier comprising the device of claim 13, wherein at least a portion of the Si-rich silicon nitride film defines at least a portion of a waveguide apt to propagate an optical radiation.

18. A method for manufacturing a device, the method comprising the steps of:

depositing by plasma enhanced deposition a Si-rich silicon nitride film over a substrate, the Si-rich silicon nitride film having a refractive index at 1.55 μm greater than or equal to about 2.1, wherein a temperature of the substrate during the deposition is greater than about 350⁰C; thermally annealing the Si-rich silicon nitride film at a temperature selected from a range of400 ^oC to l200 °C; and

coupling an electron injector and a hole injector to the Si-rich silicon nitride film so as to allow bipolar injection of electrons and holes into the Si-rich silicon nitride film.

19. The method of claim 18, wherein plasma enhanced deposition comprises plasma enhanced chemical vapor deposition.

20. The method of claim 18, wherein the annealing temperature is selected from a range of 500 ⁰C to 900 ⁰C.

21. The method of claim 18, wherein an annealing duration is selected from a range of 1 minute to 5 hours.

22. The method of claim 18, further comprising incorporating rare earth ions into the Si-rich silicon nitride film.

23. A method for manufacturing a device, the method comprising the steps of: sputtering a Si-rich silicon nitride film over a substrate, the Si-rich silicon nitride film having a refractive index at 1.55 μm greater than or equal to about 2.1;

thermally annealing the Si-rich silicon nitride film at a temperature selected from a range of 400 ^oC to l200 °C; and

coupling an electron injector and a hole injector coupled to the Si-rich silicon nitride film so as to allow bipolar injection of electrons and holes into the Si-rich silicon nitride film.

24. The method of claim 23, wherein the annealing temperature is selected from a range of 500 ⁰C to 900 ⁰C.

25. The method of claim 23, wherein an annealing duration is selected from a range of 1 minute to 5 hours.

26. The method of claim 23, further comprising incorporating rare earth ions into the Si-rich silicon nitride film.

27. A method for amplifying light, comprising the steps of

propagating an optical radiation having a wavelength selected from a range of about 1 micrometer to about 3 micrometers along a waveguide comprising a Si-rich silicon nitride material having a refractive index at 1.55 μm greater than or equal to about 2.1, and comprising silicon nanoclusters having a density greater than or equal to about 10¹⁷ cm^"3 and a rare earth material; and

injecting holes and electrons into the Si-rich silicon nitride material so that energy transfer occurs between silicon nanoclusters and erbium ions, thereby amplifying the optical radiation.