Abstract:
An electroluminescent material slot waveguide generates light in response to current injection. In one embodiment, the waveguide is formed as part of an optical resonator, such as ring resonator waveguide or distributed Bragg reflector with an anode and cathode for electrical stimulation. A compact, electrically-driven resonant cavity light emitting devices (RCLED) for Si microphotonics may be formed. Several different rare-earth ions, such as erbium, terbium and ytterbium, can be used to dope SiO2 in order to emit light at different wavelengths.

Description:
RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application Ser. No. 60/735,736 (entitled LIGHT EMITTING SLOT-WAVEGUIDE DEVICE, filed Nov. 10, 2005) and to U.S. Provisional Application Ser. No. 60/735,313 (entitled LIGHT EMITTING SLOT-WAVEGUIDE DEVICE, filed Nov. 11, 2005) which are both incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to light emitting devices, and in particular to a light emitting slot-waveguide device. 
     BACKGROUND OF THE INVENTION 
     Recent breakthroughs have boosted the interest in silicon based microphotonics as a technology for integrating optical and electronic components on a single silicon chip. In particular, the demonstration of a continuous-wave optically-pumped Si laser has been of special relevance. However, such a device is optically-pumped and emits at 1.686 μm wavelength, limiting its practical applications. 
     SUMMARY OF THE INVENTION 
     An electroluminescent doped slot waveguide generates light in response to current injection. In one embodiment, the waveguide is formed as part of an optical resonator, such as ring waveguide or distributed Bragg reflector. A compact, electrically-driven resonant cavity light emitting devices (RCLED) for Si microphotonics may be formed. In one embodiment, several different rare-earth ions, such as erbium, terbium and ytterbium, can be used to dope SiO2 in order to emit light at different wavelengths. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
         FIG. 1A  is a schematic top view of a light emitting slot waveguide device in accordance with an example embodiment. 
         FIG. 1B  is a schematic cross section view of the light emitting slot waveguide device of  FIG. 1B . 
         FIG. 2  is a graph illustrating an optical field distribution for quasi-TE optical mode perpendicular to the slot for the light emitting slot waveguide device of  FIG. 1A . 
         FIG. 3  is a graph illustrating an optical field distribution for quasi TE optical mode in a bent slot waveguide turning to the left according to an example embodiment. 
         FIG. 4  is a graph illustrating spectral transmittance of the light emitting slot waveguide device of  FIG. 1A . 
         FIG. 5A  is a graph illustrating two dimensional distribution of a DC electric field for an applied voltage. 
         FIG. 5B  illustrates a profile of the applied electric field in  FIG. 5A . 
         FIG. 6A  is a schematic cross-sectional view of a horizontal slot waveguide according to an example embodiment. 
         FIG. 6B  illustrates transverse electric field amplitude of the quasi-TM optical mode for the device in  FIG. 6A . 
         FIG. 7  is a perspective schematic view of a Fabry-Perot micro-cavity based slot-waveguide according to an example embodiment. 
         FIG. 8A  is a schematic cross section of a further alternative light emitting slot-waveguide device according to an example embodiment. 
         FIG. 8B  is a schematic perspective view of the light emitting slot-waveguide device of  FIG. 8A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following description is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims. 
     A silicon in-plane micron-size electrically-driven resonant cavity light emitting device (RCLED) is based on slotted waveguide. The device consists of a micro-ring resonator formed by a Si/SiO 2  slot-waveguide with a low-index electroluminescent material (such as Erbium-doped SiO 2  or other types of electroluminescent materials including rare earth metals) in the slot region. The geometry of the slot-waveguide permits the definition of a metal-oxide-semiconductor (MOS) configuration for the electrical excitation of the active material. Simulations predict a quality factor Q as high as 33,000 for a 40-μm-radius electrically-driven micro-ring RCLED capable to operate at a very low bias current of 1.5 nA. Lasing conditions are also discussed. 
     An electrically-driven Si light emitting device (LED) is desirable since it can be considered as the natural interface between photonics and electronics such as CMOS technology. In addition, emission at approximately 1.5-μm-wavelength is also desirable for applications in the telecommunication field. 
     Si LEDs based on metal-oxide-semiconductor (MOS) structures with Er implanted in the thin gate oxide have shown external quantum efficiencies as high as 10%, which is comparable to that of standard III-V semiconductors LEDs. By current injection through the MOS structure, energetic (hot) electrons can excite Er ions by impact ionization and generate electroluminescence at 1.54 μm. 
     An optical cavity can enhance the external quantum efficiency of LEDs and it is an essential element for a laser. In order to be employed with the aforementioned Er-doped SiO 2  active material for on-chip applications, an optical cavity should: 1) permit electrical injection, 2) present a high optical mode-active material overlap, 3) be made of CMOS-compatible materials, 4) be micron-size, and 5) exhibit a high quality-factor Q. A planar waveguide-based cavity, such as a ring or disk resonator, can provide a long light-matter interaction path. However, Er-doped SiO 2  has low refractive index and, therefore, a conventional strip waveguide using this material as the core would present two important drawbacks: a) waveguides would require a large cross-section area, which makes difficult current injection through the thick oxide, and b) the low-index-contrast system SiO 2 /air does not facilitate miniaturization. 
     A novel guided-wave slot structure, enables concentration of a large fraction of the guided mode into a thin low-index layer (slot) sandwiched between two high-index strips. In on embodiment, two doped Si strips (electrodes), sandwich a thin Er-doped-SiO 2  slot layer (gate oxide). Current injection through the gate oxide results in generation of light in the oxide-slot region where the guided-mode is strongly confined. In one embodiment, 50-μm-diameter high-Q (˜20,000) optical resonator in silicon-on-insulator based on slot-waveguides had losses as low as 10 dB/cm. Using these advantageous characteristics of the slot-waveguide geometry, compact electrically-driven resonant cavity light-emitting devices (RCLED) for Si microphotonics are obtained. 
       FIG. 1A  shows a schematic diagram of a MOS slot waveguide light emitting device  100 . It consists of a micro-ring resonator formed by a slot-waveguide  110 , also referred to as a body. In one embodiment, the slot-waveguide  110  includes a pair of high index of refraction rings of silicon  115 ,  116  that sandwich a concentric ring  120  of Er-doped-SiO 2 , referred to as the slot layer  120 . The slot layer  120  has a relatively low index of refraction compared to the high index Si rings  115 ,  116 . 
     In one embodiment, anode sections  130 ,  135  are formed outside of the slot-waveguide  110 , and a cathode  140  is formed inside the slot-waveguide  110 . In one embodiment, both the anode  135  and cathode  140  are p+ doped. One or more waveguides  150 ,  152  may be formed adjacent the slot-waveguide  110  such that they are optically coupled to the slot-waveguide  120  for providing light output from the slot-waveguide. The positioning of the anode and cathode areas may be altered such that they still provide current injection via the slot layer  120  when current flows from anode to cathode. SiO 2  may be used to cover the whole device as shown at  160 . 
     A schematic cross-section of the slot-waveguide  110  forming the ring is illustrated in  FIG. 1B . A silicon-on-insulator (SOI) platform comprises a silicon substrate  210  with a buried oxide layer  215 . In one example embodiment, a 60-nm-wide Er-doped-SiO 2  region (slot)  120  is sandwiched between two 300-nm-tall and 180-nm-wide p-type doped (p=10 18  cm −3 ) Si stripes corresponding to rings  115 ,  116 . Thin 50-nm-thick slabs or strips  220 ,  222  are introduced for defining highly doped p-type (p=10 19  cm −3 ) Si regions corresponding to anodes  130 ,  135  and cathode  140 . The slabs  220  and  222  provide some separation between the silicon rings and electrodes in one embodiment. 
     The optical mode characteristics of the slot-waveguide may be calculated by employing a beam propagation method (BPM). The transmission characteristics of an example bus-coupled micro-ring were calculated by using the transfer matrix method. The refractive indexes of undoped Si and SiO 2  (and Er-doped SiO 2 ) were assumed to be 3.48 and 1.46, respectively. The real refractive index and absorption coefficient of the doped Si regions due to the free-carrier dispersion are calculated by using the relations:
 
Δ n=Δn   e   +Δn   h =−[8.8×10 −22   ·ΔN+ 8.5×10 −18 ·(Δ P ) 0.8 ]  (1)
 
Δα=Δα e +Δα h =8.5×10 −18   ·ΔN+ 6.0×10 −18   ·ΔP   (2)
 
where
         Δn e  is the refractive index change due to electron concentration change;   Δn h  is the refractive index change due to hole concentration change;   ΔN is the electron concentration change in cm −3 ;   ΔP is the hole concentration change in cm −3 ;   Δα e  (in cm −1 ) is the absorption coefficient variations due to ΔN;   Δα h  (in cm −1 ) is the absorption coefficient variation due to ΔP.       

     A two-dimensional (2-D) semiconductor device modeling software, ATLAS from SILVACO, was employed to calculate the DC electric-field across the gate oxide of the biased structures. 
     Optical Characteristics 
       FIG. 2  illustrates the optical field distribution for the quasi-TE (major E-field component perpendicular to the Si/slot interface) for an example slot-waveguide constructed in accordance with  FIGS. 1A and 1B . Results shown and described in the present application are not represented as average, best case or worse case. They are simply results obtained from one or more example devices constructed in accordance with the described embodiments. The operating wavelength was 1.54 μm. The optical field is strongly confined in the low-index slot region  120 . The maximum normalized power in the slot  120  (with respect to the total power in the waveguide) was estimated to be around 30%. The effective refractive index was calculated to be n eff =1.9659+j9.24×10 −6 . The imaginary part (absorption constant) of n eff  represents an absorption coefficient of 3.2 dB/cm. Note that the latter value is smaller than that exhibited by the doped (p=10 18  cm −3 ) Si rings (6 dB/cm). This is because only a small fraction of the optical mode is located in the highly lossy Si regions, as revealed in  FIG. 2 . This is a unique feature of the slot-waveguide that enables the use of high-index lossy materials (for example, for defining electrodes) without introducing excessive optical losses, which is especially useful in the design of high performance electro-optic devices. 
       FIG. 3  shows the quasi-TE optical mode distribution in a bent slot-waveguide turning to the left (−x axis) with a radius of curvature of 40 μm. It is seen that the optical field is still strongly concentrated in the slot region and slightly shifts to the right side (+x axis) due to the bending effect. The effective refractive index of the bent slot-waveguide was calculated to be n eff,bend =1.9666+j9.99×10 −6 , which corresponds to an absorption coefficient of α bend =3.5 dB/cm. In addition to losses due to free-carrier absorption, radiation loss (α rad ) associated with the bend must be considered. BPM simulations revealed radiation loss of 2.9 dB/cm for a radius of curvature of 40 μm. 
     In order to estimate the performance of an example micro-ring resonator  110  illustrated in  FIG. 1A , the following parameters were used: radius (R)=40 μm, power-coupling coefficient (|κ| 2 )=0.025, and optical losses α=α scattering +α bend +α rad . α scattering  represents the optical losses in the slot-waveguide due to scattering at the sidewalls of the Si rails, which has been experimentally determined to be ˜10 dB/cm. Thus, α=16.4 dB/cm and the total internal loss in the ring, A i =α2πR=0.41 dB. The ring radius should satisfy the condition 2πR=m(λ emission /n eff,bend ), where m is an integer, in order to have a resonance at the emission wavelength λ emission =1540 nm. 
       FIG. 4  shows the transmittance characteristics of the micro-ring  110 . The calculated quality factor Q, defined as the ratio of the resonance frequency (ω r ) to the full width at half maximum of the resonance (Δω), is Q=ω r /Δω=3.3×10 4 . This value is two orders of magnitude higher than that exhibited by vertical Fabry-Perot cavities formed by multilayer Si/SiO 2  Distributed Bragg Reflectors. Since the emission wavelength, which corresponds to the Er ions sharp luminescence, is in resonance with the cavity mode, the emitted light can be enhanced by orders of magnitude. Note also that the calculated Q corresponds to a passive ring; if optical gain is achieved in the active material a narrower resonance peak, and higher Q, could be obtained. 
     Laser oscillation may occur if the following condition is satisfied: a(1−|κ| 2 )=1, where a is the inner circulation factor. For |κ| 2 =0.025, a=1.0256, which corresponds to a net optical gain of 8.64 dB/cm. Since the internal loss is α=16.4 dB/cm, the total optical gain needed for lasing would be 25 dB/cm. At present, the material system Er 3+  in SiO 2  has exhibited optical gain when optically pumped, and the maximum total gain achieved so far is smaller than the calculated of 25 dB/cm. Lasing may be obtained with further reductions in waveguide losses through improvements in the processing of the slot-waveguides in order to reduce scattering, which is estimated to be the main source of loss in the proposed structure. 
     Electrical Characteristics 
       FIG. 5  shows the 2-D distribution of the dc electric field for an applied voltage (V anode −V cathode ) of 20 V. The transverse electric field in the slot region is nearly uniform and most of the applied voltage drops across the Er-doped SiO 2 . This assures a uniform current injection through the gate oxide. The high conductivity of the doped (p=10 18  cm −3 ) Si strips  220 ,  222  permits placement of the lossy electrode regions (p=10 19  cm −3 )  130 ,  135 , and  140  far from the waveguide core, reducing significantly the optical losses of the waveguide. Carrier transport through the gate oxide in the studied devices can be attributed to Fowler-Nordheim (F-N) tunneling. Assuming an experimental value of 2 mA/cm 2  for the F-N current density needed to produce electroluminescence saturation in Er-doped SiO 2  MOS devices, the bias current for the slot-waveguide ring LED would be I=J·A ring =(2 mA/cm 2 )·(2π40 μm0.3 μm)=1.5 nA, where A ring  is the area of the vertical surface of the active region (slot  120 ). Thus, if the needed voltage to achieve such a current density is 20 V, the power consumption would be only 30 nW. This small power consumption arises from the small area of the active area. 
     Other Configurations 
     Besides the vertical slot-waveguide configuration of  FIGS. 1A and 1B , other configurations may be used such as a horizontal configuration  600  shown in  FIG. 6A . The horizontal configuration  600  is formed on an oxide layer  605  formed on a silicon substrate  610  in one embodiment. A first silicon ring  615  is formed and supports a slot  620 , followed by a second silicon ring  625 . Slabs  630  are formed on either side of the first silicon ring and the cathode  635  is also formed spaced from the rings. Further slabs  640  are formed on either side of the second silicon ring  625  along with anode  645 . 
     In the horizontal configuration  600 , the device would operate under quasi-TM polarization (major E-field component perpendicular to the Si/slot interface), as illustrated in  FIG. 6B . The complex refractive index of the slot-waveguide shown in  FIG. 6A  was calculated to be n eff =2.1198+j1.11×10 −5 , which corresponds to an absorption coefficient of 3.9 dB/cm. This value is slightly larger than that exhibited by the vertical configuration due to the presence of more doped Si regions. The horizontal configuration can be advantageous in order to reduce scattering losses induced by imperfections at the Si/SiO 2  interfaces. This is because a horizontal slot-waveguide could be fabricated by ion implantation (oxygen and erbium ions), deposition or lateral overgrowth epitaxial techniques, which would lead to much smoother interfaces than that produced by reactive ion etching techniques, used for the fabrication of vertical slot-waveguides. 
     Besides a micro-ring resonator, a Fabry-Perot (F-P) microcavity defined by DBRs, such as that shown schematically in  FIG. 7  may also be utilized for optical feedback. In this embodiment, a slot waveguide  710  is formed substantially straight with a slot portion  712  sandwiched by silicon portions  713  and  714 . An anode  715  and cathode  720  are disposed on either side of the slot waveguide  710 . Distributed Bragg Reflectors  725  and  730  are formed on both ends of the slot waveguide  710 . Similar F-P cavities based on conventional strip photonic wire have been demonstrated on SOI substrates, exhibiting Q&gt;1400. 
       FIGS. 8A and 8B  illustrate different views of a light emitting disk resonator  800  formed by a horizontal (stacked layers) Er-doped slot structure. In one embodiment, an insulating layer  810  is formed of SiO 2  or other material, and supports an undoped Si layer  815 . A p++ doped ring anode  820  is formed in the Si layer  815 , and an Er doped SiO 2  disk  825  is formed surrounded by the ring anode  820 . An n+ doped polysilicon layer  830  is then formed on top of the disk  825 , followed by formation of an n++ doped cathode ring  835  supported by the n+ doped polysilicon layer  830 . In one embodiment, the cathode ring  835  is supported by the outer top portion of the n+ doped polysilicon layer  830 . The n+ doped polysilicon layer  830  and Si layer  815 , sandwich the Er doped disk  825 , creating a light emitting slot waveguide. Injection currents created by a voltage across the anode  820  and cathode rings induce the Er doped SiO 2  disk  825  to emit light. 
     The Abstract is provided to comply with 37 C.F.R. § 1.72(b) to allow the reader to quickly ascertain the nature and gist of the technical disclosure. The Abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.