Microresonantor systems and methods of fabricating the same

Various embodiments of the present invention are related to microresonator systems and to methods of fabricating the microresonator systems. In one embodiment, a microresonator system comprises a substrate having a top surface layer and at least one waveguide embedded in the substrate and positioned adjacent to the top surface layer of the substrate. The microresonator system also includes a microresonator having a top layer, an intermediate layer, a bottom layer, a peripheral region, and a peripheral coating. The bottom layer of the microresonator is attached to and in electrical communication with the top surface layer of the substrate. The microresonator is positioned so that at least a portion of the peripheral region is located above the at least one waveguide. The peripheral coating covers at least a portion of the peripheral surface and has a relatively lower index of refraction than the top, intermediate, and bottom layers of the microresonator.

TECHNICAL FIELD

Embodiments of the present invention are directed to microresonator systems, and, in particular, to microresonator systems that can be used as lasers, modulators, and photodetectors and to the methods for fabricating these systems.

BACKGROUND

In recent years, the increasing density of microelectronic devices on integrated circuits has lead to a technological bottleneck in the density of metallic signal lines that can be used to interconnect these devices. In addition, the use of metallic signal lines yields a significant increase in power consumption and difficulties with synchronizing the longest links positioned on top of most circuits. Rather than transmitting information as electrical signals via signal lines, the same information can be encoded in electromagnetic radiation (“ER”) and transmitted via waveguides, such as optical fibers, ridge waveguides, and photonic crystal waveguides. Transmitting information encoded in ER via waveguides has a number of advantages over transmitting electrical signals via signal lines. First, degradation or loss is much less for ER transmitted via waveguides than for electrical signals transmitted via signal lines. Second, waveguides can be fabricated to support a much higher bandwidth than signal lines. For example, a single Cu or Al wire can only transmit a single electrical signal, while a single optical fiber can be configured to transmit about 100 or more differently encoded ER.

Recently, advances in materials science and semiconductor fabrication techniques have made it possible to develop photonic devices that can be integrated with electronic devices, such as CMOS circuits, to form photonic integrated circuits (“PICs”). The term “photonic” refers to devices that can operate with either classically characterized ER or quantized ER with frequencies that span the electromagnetic spectrum. PICs are the photonic equivalent of electronic integrated circuits and may be implemented on a wafer of semiconductor material. In order to effectively implement PICs, passive and active photonic components are needed. Waveguides and attenuators are examples of passive photonic components that can typically be fabricated using conventional epitaxial and lithographic methods and may be used to direct the propagation of ER between microelectronic devices. However, these fabrication methods often produce defects in the photonic components that can result in significant channel loss. One common source of loss is scattering due to surface roughness.

FIG. 1shows a top view of an example microdisk102. In general, because a microdisk has a larger index of refraction than its surroundings, channels become trapped as a result of total internal reflection near the circumference of the microdisk and may be trapped within the microdisk. Modes of ER trapped near the circumference of the microdisk are called “whispering gallery modes (‘WGMs’).” A directional arrow104located near the circumference of microdisk102represents a hypothetical WGM propagating near the circumference of microdisk102. Intensity plot106shows intensity of the WGM versus distance along line A-A of microdisk102. Dashed-line intensity curves108and110show the WGM confined substantially to a peripheral region of microdisk102. Portions of curves108and110that extend beyond the diameter of microdisk102represent evanescence of the WGM along the circumference of microdisk102. However, enlargement112of an edge of microdisk102shows surface roughness which can be caused by an etching process used to form microdisk102. This surface roughness increases scattering loss and reduces the Q factor of microdisk102. Physicists and engineers have recognized a need for photonic components designs and fabrication methods that reduce scattering losses and increase Q factors associated with photonic components.

SUMMARY

Various embodiments of the present invention are related to microresonator systems and to methods of fabricating the microresonator systems. In one embodiment of the present invention, a microresonator system comprises a substrate having a top surface layer and at least one waveguide embedded in the substrate and positioned adjacent to the top surface layer of the substrate. The microresonator system also includes a microresonator having a top layer, an intermediate layer, a bottom layer, a peripheral region, and a peripheral coating. The bottom layer of the microresonator is attached to and in electrical communication with the top surface layer of the substrate. The microresonator is positioned so that at least a portion of the peripheral region is located above the at least one waveguide. The peripheral coating covers at least a portion of the peripheral surface and has a relatively lower index of refraction than the top, intermediate, and bottom layers of the microresonator.

DETAILED DESCRIPTION

Various embodiments of the present invention are directed to microscale resonator (“microresonator”) systems and to methods of fabricating the microresonator systems. The microresonator systems may be used as lasers, modulators, and photodetectors and can be incorporated with CMOS circuitry. In the various microresonator and fabrication embodiments described below, a number of structurally similar components comprising the same materials have been provided with the same reference numerals and, in the interest of brevity, an explanation of their structure and function is not repeated.

FIG. 2Ashows an isometric view of a microresonator system200in accordance with embodiments of the present invention. Microresonator system200comprises a microdisk202attached to a top surface layer204of a substrate206, a first electrode208adjacent to a top surface210of microdisk202, and a second electrode212attached to top surface layer204and positioned adjacent to microdisk202. Microdisk202is a microresonator of microresonator system200and can be configured to support certain WGMs. Substrate206includes two waveguides214and216that extend through substrate206and are positioned adjacent to top surface layer204. Waveguides214and216are located beneath peripheral regions of microdisk202. Microdisk202comprises a top layer218, a bottom layer220, and an intermediate layer222sandwiched between top layer218and bottom layer220. Bottom layer220can be comprised of the same material as top surface layer204, as described below with reference toFIG. 2B. Layers218,220, and222of microdisk202are described in greater detail below with reference toFIG. 3. Microresonator system200includes a relatively thin peripheral coating224covering at least a portion of the peripheral surface of microdisk202.

FIG. 2Bshows a cross-sectional view of microresonator system200along a line2B-2B, shown inFIG. 2A, in accordance with embodiments of the present invention. As shown inFIG. 2B, waveguides214and216are located beneath portions of peripheral regions of microdisk202. Second electrode212is in electrical communication with bottom layer220via top surface layer204. Although only a single second electrode212is positioned on top surface layer204, in other embodiments of the present invention, two or more electrodes can be positioned on top surface layer204.

Top layer218can be a III-V semiconductor doped with electron donor impurities that produce a larger electron than hole concentrations. These semiconductors are referred to as “n-type semiconductors.” Bottom layer220can be a III-V semiconductor doped with element acceptor impurities that produce a larger hole than electron concentration. The semiconductors are referred to as “p-type semiconductors.” Note that Roman numerals III and V refer to elements in the third and fifth columns of the Periodic Table of the Elements. Intermediate layer222includes one or more quantum wells. Each quantum well can be a relatively thin, III-V semiconductor layer sandwiched between two layers of a different type of III-V semiconductor.FIG. 3Ashows a cross-sectional view of layers comprising an exemplary microdisk202in accordance with embodiments of the present invention. InFIG. 3A, top layer218can be p-type InP, where Zn can be used as the dopant, and bottom layer220can be n-type InP, where Si can be used as the dopant. Intermediate layer222includes three quantum wells301-303of InxGa1−xAsyP1−y, where x and y range between 0 and 1. Intermediate layer222also includes barrier layers305-308of InxGa1−xAsyP1−y, where x and y range between 0 and 1. The choice of compositions x and y are well-known in the art. For example, for layers which are latticed matched to InP layers218and220, the x value is chosen to be 0.47. The choice of y determines the bandgap energy of the quantum well. Operation of a quantum well is described below with reference toFIG. 6A. The quantum wells301-303can be configured to emit ER at a desired wavelength λ while the barrier layers305-308can be configured to have a relatively larger bandgap in order to confine carriers (i.e., electrons and holes) injected into the quantum well. Layers306and307separate quantum wells301-303, and layers305and308are two relatively thicker layers that separate quantum wells301and303from layers218and220, respectively. Peripheral coating224can be an un-doped phosphorus-based semiconductor, such as InP. Substrate206can be comprised of SiO2, Si3N4or another suitable dielectric insulating material. Waveguides214and216can be comprised of a column IV element, such as Si and Ge. In other embodiments of the present invention, other suitable III-V semiconductor, such as GaAs, GaP or GaN, may be used.

The thickness of peripheral coating224can range from about 5 to about 25 nm or range from about 10 to about 20 nm and have a relatively lower index of refraction than the index of refraction associated with layers218,220, and222of microdisk202. In addition, peripheral coating224also has a smoother outer surface than the outer surface of layers218,220, and222. As a result, peripheral coating224serves as a cladding layer that also reduces the amount of loss due to scattering along the periphery of microdisk202, which, in turn, results in a higher associated Q factor.FIG. 3Bshows a cross-sectional view of a WGM located in a peripheral region of microdisk202in accordance with embodiments of the present invention. As shown inFIG. 3B, dashed-line ellipses310and312identify portions of the peripheral region of microdisk202occupied by a WGM and show evanescent coupling of the WGM into waveguides214and216.

The microresonators of the microresonator system embodiments of the present invention are not limited to circular-shaped microdisks, such as microdisk202. In other embodiments of the present invention, the microresonators can be microrings or other suitable microresonators, and the microresonators can be circular, elliptical, or have any other shape that is suitable for creating resonant ER.FIG. 4Ashows an isometric view of a second microresonator system400in accordance with embodiments of the present invention. Microresonator system400comprises a microring402attached to top surface layer204of substrate206and is positioned so that portions of microring402are located above waveguides214and216. A microring electrode404is located on the top surface of microring402, and microring402comprises top, intermediate, and bottom layers having the same composition as layers218,220, and222of microdisk202. The bottom layer of microring402may be comprised of the same material as top surface layer204. Microresonator system400also includes a relatively thin peripheral coating406covering the peripheral surface of microring402.

FIG. 4Bshows a cross-sectional view of second microresonator system400along a line4B-4B, shown inFIG. 4A, in accordance with embodiments of the present invention. As shown inFIG. 4B, waveguides214and216are located beneath portions of microring402. Second electrode212is in electrical communication with the bottom layer of microring402via top surface layer204. In other embodiments of the present invention, two or more electrodes can be positioned on top surface layer204.

FIGS. 5A-5Jshow isometric and cross-sectional views that are associated with a method of fabricating photonic system200, shown inFIG. 2, in accordance with embodiments of the present invention.FIG. 5Ashows an isometric view of a first structure500comprising a top layer502, an intermediate layer504, a bottom layer506, and an etch stop layer508supported by a phosphorus-based wafer510. Layers502and506can be comprised of n-type and p-type III-V semiconductors, such as InP or GaP doped with Si and Zn, respectively. Intermediate layer504includes at least one quantum well, as described above with reference toFIG. 3. Etch stop layer508can be a thin layer of latticed matched In0.53Ga0.47As. Layers502,504, and506can be deposited using molecular beam expitaxy (“MBE”), liquid phase epitaxy (“LPE”), hydride vapor phase epitaxy (“HVPE”), metalorganic vapor phase expitaxy (“MOVPE”), or another suitable expitaxy method.FIG. 5Bshows a cross-sectional view of layers502,504,506,508, and wafer510.

Next, as shown in the cross-sectional view ofFIG. 5C, sputtering can be used to deposit an oxide layer512over top layer502. Oxide layer512can be used to facilitate wafer bonding of top layer502onto substrate206, as described below with reference toFIG. 5G. Layer512can be SiO2, Si3N4, or another suitable dielectric material that substantially enhances wafer bonding to substrate206.

FIG. 5Dshows a silicon-on-insulator substrate (“SOI”) wafer514having a Si layer516on an oxide substrate layer518. Silicon waveguides214and216can be fabricated in Si layer516as follows. A photoresist can be deposited over Si layer516and a photoresist mask of waveguides214and216can be patterned in the photoresist using UV lithography. Waveguides214and216can then be formed in Si layer514using a suitable etch system, such as inductively coupled plasma etcher (“ICP”), and a low-pressure, high-density etch system with a chemistry based on Cl2HBr/He/O2. After waveguides214and216have been formed in Si layer516, a solvent can be used to remove the photoresist mask leaving waveguides214and216, as shown inFIG. 5E. An oxide layer comprised of the same oxide material as substrate518can be deposited over waveguides214and216using liquid-phase, chemical-vapor deposition. Chemical mechanical polishing (“CMP”) processes may be used to planarize the deposited oxide in order to form substrate206with embedded waveguides214and216, as shown in the cross-sectional view of substrate206inFIG. 5F.

Next, as shown inFIG. 5G, first structure500is inverted and wafer bonding is used to attach oxide layer512to the top surface of substrate206. Selective wet etching can be used to remove layer510in order to obtain a second structure520shown inFIG. 5H. Etch stop layer508can included to stop the etching process from reaching layer506. Hydrochloric acid can also be used to remove an InP-based wafer510because there is an etch selectivity between the InP and the InGaAs of etch stop layer508.

Next, reactive ion etching (“RIE”), chemically assisted ion beam etching (“CAIBE”), or inductively coupled plasma (“ICP”) etching can be used to etch layers502,504, and506into the form of microdisk202, as shown inFIG. 5I. Note RIE, CAIBE, and ICP can also be used to form a microring as shown inFIG. 4or another suitable microresonator shape. A portion of layer502adjacent to substrate206is left in order to from top surface layer204. The resulting microdisk202structure has a rough outer surface resulting from the etching process. The dry etch process causes a thin damaged region on the surface of microdisk202due to bombarding of the surface of microdisk202with reactive elements. This damaged and roughened outer surface of layers218,220, and222causes excess loss and a lower Q of ER confined to these layers. Mass transport process is used to smooth out this rough surface while annealing out the damage.

After microdisk202is formed by etching, microdisk202and substrate206are placed in a reaction chamber filled with a suitable partial pressure of phosphine gas (PH3) and hydrogen H2, and the chamber is heated to a temperature ranging from about 400° C. to about 700° C. Indium atoms in layers218,220, and222become dissociated and through mass transport are able to react with phosphorus in the phosphine gas at the outer surface of microdisk202forming a relatively thin peripheral coating224of InP covering the outer surface of microdisk202. Mass transport results in the erosion of sharp convex surfaces and the filling in of concave surfaces due to surface energy minimization and diffusion.

FIG. 5Jshows peripheral coating224covering microdisk202in accordance with embodiments of the present invention. Peripheral coating224may have a thickness ranging from about 5 nm to about 25 nm or from about 10 nm to about 20 nm and may serve as a cladding layer because the index of refraction of peripheral coating224is relative lower than the index of refraction of the layers comprising microdisk202. In addition, as shown in an enlargement524of the outer surface of microdisk202, peripheral coating224provides a smoother outer surface526that reduces the amount of scattering created by rough outer surface528and increases the Q factor associated with microdisk202. The mass transported InP in peripheral coating224has a wider bandgap than the quantum wells of quantum well layer222and reduces loss due to surface recombination of the carriers at the defects on the etched surface of microdisk202. This “heterointerface” introduces a built in field which prevents the injected carriers from reaching the surface of microdisk202.

Microdisk202can be used as a laser that generates coherent ER transmitted in waveguides214and216. A laser comprises three basic components: a gain medium or amplifier, a pump, and feedback of the ER inside an optical cavity. The quantum wells of intermediate layer222comprise the gain medium, an externally applied current or voltage to electrodes208and212is the pump, and feedback is created by total internal reflection as a WGM generated by pumping quantum wells of intermediate layer222propagates near the circumference of microdisk202.

A gain medium can be comprised of at least one quantum well with a suitable bandgap. The quantum well size and the bulk material surrounding the quantum well determine the energy level spacing of electronic states in the quantum well. Typically, the quantum well is configured to have a relatively small number of quantized electronic energy levels in the valance band and a few quantized hole energy levels in the conduction band. Electrons transitioning from the lowest energy levels in the conduction band to energy levels in the valance band determine the emission wavelength λ of the gain medium.FIG. 6Ashows an energy level diagram600associated with quantized electronic energy states of a quantum well-based gain medium of width α. Narrower region602with bandgap energy Egcorresponds to a quantum well, and relatively wider regions604and606with bandgap energy Ēgcorrespond to bulk material surrounding the quantum well. As shown inFIG. 7A, the quantum well has a hole energy level608in the conduction band and three electronic energy levels610-612in the valence band. Because the gain medium comprises semiconductor material, an appropriate electronic stimulus, such as electrical pumping, promotes electrons from the valance band into the quantized levels in the conduction band, such as hole energy level608. Spontaneous recombination of an electron in the conduction with a hole in the valance band results in emission of a photon having an energy given by hc/λ, where h is Plank's constant, and c is the speed of ER in a vacuum. A stimulated emission occurs as a result of photons in the WGM stimulating the gain medium to generate more photons at the same energy or wavelength. In both spontaneous and stimulated radiative emissions, the energy of the ER emitted is:

E2-E1=hcλ
where E2is the energy level608of the electrons that have been pumped into the conduction band, and E1is the energy level610associated with holes in the valance band that combine with electron from the conduction band. As long as the electrical pump is applied to the gain medium, feedback caused by total internal reflection within microdisk202causes the intensity of the WGM to increase. Lasing occurs when the gain equals the loss inside microdisk202. Microdisk202forms the optical cavity with gain, and the waveguides214and216couple the ER out of microdisk202.

FIG. 6Bshows a schematic representation of the microresonator system200, shown inFIG. 2, operated as a laser in accordance with embodiments of the present invention. As shown inFIG. 6B, first and second electrodes208and212are connected to a current source614. Quantum-wells of microdisk202can be operated as a gain medium by pumping microdisk202, as described above with reference toFIG. 6A, with a current of an appropriate magnitude supplied by current source614. As a result, a WGM having a wavelength λ is generated within microdisk202, and total internal reflection causes the WGM to propagate near the circumference of microdisk202as the intensity of the WGM increases. The WGM evanescently couples into the waveguides214and216yielding ER with a wavelength λ that propagates in waveguides214and216.

FIG. 7Ashows a schematic representation of the microresonator system200, shown inFIG. 2, operated as a modulator in accordance with embodiments of the present invention. Current source614is connected to a data source702, which can be a central processing unit, memory, or another data generating device. ER source704is coupled to waveguide216and emits ER with a substantially constant intensity over time, as shown inFIG. 7B. Returning toFIG. 7A, the amount of ER coupled into microdisk202depends on the detuning, the coupling coefficient, and the losses inside of microdisk202. When the wavelength λ of the ER emitted by source704is detuned from the resonance of microdisk202, the ER does not couple from waveguide216into microdisk202. When the wavelength λ of the ER is at resonance with microdisk202, the transmission of the ER propagating in the waveguide216is reduced because the ER is evanescently coupled into microdisk202creating a WGM. A portion of the ER transmitted in waveguide216evanescently couples into the peripheral region of microdisk202located above waveguide216and propagates in the peripheral region as a WGM with a wavelength λ. Data source702encodes data in the WGM by modulating the magnitude of the current generated by current source614. Modulating the magnitude of the current transmitted between electrodes208and212causes the index of refraction of microdisk202to correspondingly change. When the index of refraction of microdisk202is changed, the resonant wavelength of microdisk202changes causing a detuning from the resonant wavelength of ER transmitted in waveguide216. This in turn modulates the transmission of ER from waveguide216into microdisk202and subsequently modulates the intensity of the ER transmitted in waveguide216. When waveguide214is present, ER can be transferred to waveguide214from the input waveguide216via microdisk202. The amount of ER transferred to waveguide214depends on the coupling strength. Modulating the index of refraction of microdisk202results in a reduction in the intensity of the ER transmitted to waveguide214. One can also modulate the intensity of the ER in waveguide216by adjusting the loss inside microring202. This is accomplished by using the quantum confined stark effect which modulates the bandgap of the quantum well through the application of an applied voltage. Increasing the loss in microdisk202modulates the intensity transmitted past microdisk202in waveguides214and216.

FIG. 7Cshows intensity versus time of modulated ER where relatively lower intensity regions706and708correspond to a relatively higher index of refraction induced on microdisk202. The relative intensities can be used to encode information by assigning a binary number to the relative intensities. For example, the binary number “0” can be represented in a photonic signal by low intensities, such as intensity regions706and708, and the binary number “1” can be represented in the same photonic signal by relatively higher intensities, such as intensity regions710and712.

FIG. 8shows a schematic representation of the microresonator system200, shown inFIG. 2, operated as a photodetector in accordance with embodiments of the present invention. Modulated ERλencoding information is transmitted in waveguide216. The ER evanescently couples into the peripheral region of microdisk202producing a corresponding modulated WGM. Fluctuations in the intensity of WGMs induces a corresponding fluctuating current between first and second electrodes208and212. The fluctuating current is an electrical signal encoding the same data encoded in the modulated ER, which can be processed by computational device802.