Patent Application
20130062610
This invention relates in general to light emitting devices on silicon substrates and more specifically to crystalline reflectors in a substrate structure.
A semiconductor LED grown onto a substrate emits light in all directions. If an LED is grown onto a virtual substrate consisting of silicon and a transition layer, some of the light emitted from the LED impinges upon the substrate, a percentage of which is absorbed. Silicon typically absorbs 60% of the visible light hitting it, depending upon the wavelength above a band edge. It is therefore desirable to reflect the visible light from the LED before it reaches the silicon substrate, therefore making the LED more efficient because of improved light extraction.
Virtual substrates of the type discussed herein are lattice matched rare earth oxides that act as a transition layer to transition from a silicon substrate (wafer or portion thereof) on one side to the desired semiconductor (such as Ge, GaN, GaAs, etc.) on the other side. The key component of this design is to maintain single crystallinity from the substrate to the top semiconductor layer. These transition layers may be oxides, which are dielectrics and transmit visible light. See for example copending United States Patent Application entitled “IIIOxNy ON Single Crystal SOI Substrate and III N Growth Platform”, filed 30 Aug. 2011, bearing Ser. No. 13/221,474, and incorporated herein by reference.
One way to achieve this goal is to fabricate a Distributed Bragg Reflector (DBR) in the substrate structure, which is an alternating high-low-high-low refractive index stack tuned such that constructive interference occurs for a given wavelength in order to maximize reflection. See for example, the above described copending United States patent Application. Such an approach works well for light that is propagating normal to the plane of the wafer (and the DBR) but is less efficient at large angles from the surface normal.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved lattice matched crystalline reflector in a substrate structure.
It is another object of the present invention to provide a new and improved lattice matched crystalline reflector in a substrate structure that substantially improves LED efficiency.
It is another object of the present invention to provide new and improved methods of fabricating a lattice matched crystalline reflector in a substrate structure.
Briefly, to achieve the desired objects and aspects of the instant invention in accordance with a preferred embodiment thereof, provided is a virtual substrate structure with a lattice matched crystalline reflector for a light emitting device. The structure includes a single crystal rare earth oxide layer deposited on a silicon substrate and substantially crystal lattice matched to the silicon substrate. A reflective layer of single crystal electrically conductive material is deposited on the layer of single crystal rare earth oxide and a layer of single crystal semiconductor material is positioned in overlying relationship to the reflective layer and substantially crystal lattice matched to the reflective layer. A single crystal rare earth oxide layer is optionally deposited between the reflective layer and the layer of semiconductor material.
The desired objects and aspects of the instant invention are further realized in accordance with a specific embodiment of a virtual substrate structure with a lattice matched crystalline reflector for a light emitting device. The structure includes a first at least one layer of single crystal Gadolinium oxide (Gd2O3) deposited on the silicon substrate. The first at least one layer of single crystal Gadolinium oxide (Gd2O3) is substantially crystal lattice matched to the silicon substrate. A reflective layer of single crystal electrically conductive ytterbium is deposited on the first at least one layer of single crystal Gadolinium oxide (Gd2O3) and a second at least one layer of single crystal Gadolinium oxide (Gd2O3) is deposited on the reflective layer. The second at least one layer of single crystal Gadolinium oxide (Gd2O3) is substantially crystal lattice matched to the reflective layer. A layer of either III oxide or III nitride single crystal semiconductor material is deposited on the second at least one layer of single crystal Gadolinium oxide (Gd2O3). The layer of III nitride single crystal semiconductor material is substantially crystal lattice matched to the reflective layer.
The desired objects and aspects of the instant invention are further realized in accordance with a method of fabricating a virtual substrate structure with a lattice matched crystalline reflector for a light emitting device including the step of depositing at least one layer of single crystal rare earth oxide on a silicon substrate. The layer of single crystal rare earth oxide is substantially crystal lattice matched to the silicon substrate. Depositing a reflective layer of single crystal electrically conductive material on the at least one layer of single crystal rare earth oxide, and positioning a layer of single crystal semiconductor material in overlying relationship to the reflective layer and substantially crystal lattice matched to the reflective layer.
The foregoing and further and more specific objects and advantages of the instant invention will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment thereof taken in conjunction with the drawings, in which:
Referring to
A layer 14 of single crystal rare earth oxide (REO) or silicon is grown directly on the surface of silicon substrate 12. Throughout this disclosure whenever rare earth materials are mentioned it will be understood that “rare earth” materials are generally defined as any of the lanthanides as well as scandium and yttrium. In some applications it may be desirable to use a substrate composed of materials other than pure silicon and in these applications it will be understood that other rare earth materials that are substantially crystal lattice matched with the substrate material can be used if desired. Thus, layer 14 is relatively easily grown as a thin layer of single crystal material directly on substrate 12. The various layers of structure 10 can be grown by a variety of methods including MBE, MOCVD, PLD (pulsed laser deposition) sputtering, ALD (atomic layer epitaxy), or any other known growth method for thin films. Also, entire structure 10 or any part thereof can be grown in a single growth step in a method with less cost and easier to process or fabricate.
A single crystal electrically conductive layer 16 is deposited on layer 14 and a second layer 18 of single crystal rare earth oxide (REO) or silicon is grown directly on the surface of layer 16. Basically, layers 14, 16, and 18 form a single crystal mirror inside a rare earth oxide or silicon layer. It will be understood that in the application in which layer 18 is or includes single crystal silicon, the layer will be so thin that very little light absorption will occur. A layer 20 of light emitting single crystal semiconductor material, such as Ge, GaN, GaAs, etc., is deposited on the surface of layer 18 to serve as a virtual substrate for the later fabrication of electronic devices. Layers 14, 16, 18, and 20 are layers of single crystal material. Layers 14 and 18 may be graded from one mixture to another such that the REO layers can more precisely match the lattice constant of the material on both the top and bottom surfaces. Thus, a virtual substrate structure is provided that has the properties of a mirror as well as maintaining single crystallinity across the entire structure.
It will be understood that the mechanism of reflection from an electrical conductor's surface (e.g. conductor 16) is via interaction of the light's electric field with the mobile charge within the conductor. The electric field penetrates a characteristic skin-depth into the conductor, which is related to the frequency of light, the resistivity of the conductor, and the magnetic permeability of the conductor. Various candidate materials exist which are both conducting and may be deposited as single crystal films on either silicon or a single crystal rare earth oxide.
One possible implementation of conductor 16 is to use metals or semi-metals which are deposited as single crystal films. For example, referring to
Zirconium Diboride is a semimetal that may be deposited as a single crystal on top of silicon, and may also be deposited on rare earth oxide films, for example, as layer 16 in
Referring additionally to
In this specific example, electrically conductive layer 16 includes a thin film of ytterbium which, as stated above, is very closely matched to silicon so as to make it an ideal candidate. Layer 18 includes a single crystal rare earth oxide, which in this specific example is gadolinium oxide (Gd2O3). Gadolinium oxide layer 18 is an optional transition of dielectric material that electrically separates semiconductor layer 20 from electrically conductive layer 16 in applications where such a separation is either desired or required. Semiconductor layer 20, in this specific example is GaN but could be any other III nitride semiconductor material suitable for use as a virtual substrate where the III material is any metal selected from the III group in the periodic table. Also, gadolinium oxide layer 18 can be graded to more closely lattice match ytterbium layer 16 and GaN layer 20 or other III semiconductor material.
Turning now to
Turning now to
Turning now to
A single crystal REO layer 58 of graded REO, a rare earth oxide, in this specific example includes gadolinium oxide (Gd2O3) is deposited between electrically conductive layer 56 and a virtual substrate 60. Single crystal REO layer may be, for example, graded from crystalline ytterbium oxide (Yb2O3) adjacent layer 56 to Gadolinium oxide (Gd2O3) adjacent layer 60 to provide a closer lattice match. It should be noted that the grading can be incorporated in one of linearly or step wise. Other combinations of rare earth oxides might be used in layer 58, especially if a different reflector layer 56 is included and/or if a different III nitride material is included in layer 60.
Thus, in the present invention a new and improved virtual substrate structure with a lattice matched crystalline reflector for a light emitting device is disclosed. In all examples a virtual substrate structure is provided that has the properties of a mirror as well as maintaining single crystallinity across the entire structure. The new and improved lattice matched crystalline reflector in the substrate structure substantially improves LED efficiency. Also, new and improved methods of fabricating a virtual substrate structure with lattice matched crystalline reflector are disclosed.
Various changes and modifications to the embodiments herein chosen for purposes of illustration will readily occur to those skilled in the art. To the extent that such modifications and variations do not depart from the spirit of the invention, they are intended to be included within the scope thereof which is assessed only by a fair interpretation of the following claims.
Having fully described the invention in such clear and concise terms as to enable those skilled in the art to understand and practice the same, the invention claimed is:
