Patent Application
20070023761
Silicon carbide (SiC) is used as a substrate for a variety of electronic and optical devices. Many electronic and optical devices are formed of layers composed of one or more group III elements and nitrogen, and are generally referred to as group III nitrides. One of the group III nitrides is gallium nitride. Devices typically formed using the gallium nitride material system include, for example, transistors and light emitting devices. The gallium nitride material system includes alloys formed using various combinations of aluminum, boron, gallium, and indium with nitrogen. This includes the various binary endpoints and ternary endpoints such as aluminum nitride (AIN), gallium nitride (GaN), gallium indium nitride (GaInN), etc. Silicon carbide is a desirable substrate material for devices formed using the gallium nitride material system because it is electrically conductive. However, while having a crystal structure similar to the crystal structure of materials formed using the gallium nitride material system, the lattice constant of silicon carbide is smaller than the lattice constant of materials formed using the gallium nitride material system. The lattice mismatch between silicon carbide and materials formed using the gallium nitride material system causes dislocation defects to form in the gallium nitride material when the gallium nitride material is grown thicker than a critical thickness. These dislocations degrade the performance and reliability of gallium nitride-based devices.
Unfortunately, substrates that exhibit suitable electrical characteristics and that have a lattice constant similar to the lattice constant of materials formed using the gallium nitride material system have not been developed. Therefore, the development of devices formed in the gallium nitride material system on a silicon carbide substrate has necessitated the development of elaborate growth schemes such as epitaxial lateral overgrowth, referred to as “ELOG,” to reduce the defect density. Even after grown using a technique such as ELOG, the defect density in the gallium nitride grown on silicon carbide is still ˜106/cm−2. A substrate that is more closely lattice matched to materials formed using gallium nitride-based materials could allow the growth of gallium nitride-based materials with a lower dislocation density and improved device performance.
Embodiments in accordance with the invention provide a substrate for an electronic device formed in a group III nitride material system comprising a layer of silicon carbon and a layer of silicon carbon germanium over the layer of silicon carbon, the layer of silicon carbon and the layer of silicon carbon germanium forming a substrate for a device formed in the group III nitride material system.
Embodiments in accordance with the invention also comprise a method of forming a silicon carbon germanium substrate for an electronic device formed in a group III nitride material system. The method comprises forming a layer of silicon carbon, forming a layer of silicon carbon germanium over the layer of silicon carbon, and forming an electronic device in the group III nitride material system directly on the layer of silicon carbon germanium.
The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.
One way to decrease the lattice mismatch between the substrate and the gallium nitride-based materials is to add germanium to the silicon carbide substrate. Adding germanium to the silicon carbide substrate will increase the lattice constant of the substrate. The development of SiC:Ge alloys presents technological challenges and to date only small amounts of germanium have been incorporated into the silicon carbide crystal. X-ray diffraction data shows that the lattice constant of SiC:Ge is indeed increased compared to silicon carbide. Thin layers of SiC:Ge have been incorporated into an electronic device to form a silicon carbide heterostructure device. However, the use of germanium to form a substrate with a closer lattice-match to devices formed using a group III-nitride, and particularly the gallium nitride material system, has heretofore been unrealized.
Although the embodiments of the silicon carbon germanium substrate for a group III nitride-based device will be described in the context of forming a substrate for a gallium nitride-based device, other materials having a lattice constant similar to the lattice constant of the layers of a gallium nitride-based device can also be formed on the silicon carbon germanium substrate.
In a particular embodiment, the composition of the second substrate layer 104 is (Si0.5-xC0.5-y)Gex+y, where the germanium content varies from approximately 5% to approximately 40%. The values of x and y are chosen to provide the desired lattice constant for the subsequent device layers. In one embodiment, the second substrate layer 104 has a composition of Si0.35 C0.5Ge0.15. However, the second substrate layer 104 may have a composition that ranges between and includes Si0.45C0.5Ge0.05 and Si0.10C0.5Ge0.4. Due to the bond lengths of silicon, carbon and germanium, it is likely that the germanium will substitute for the silicon, but a portion of the germanium may also substitute for carbon. If a gallium nitride device is to be subsequently grown on the second substrate layer 104, the second substrate layer 104 should be formed with a composition of (Si0.5-xC0.5-y)Gex+y to yield an in-plane lattice constant of 3.19 Å. If an aluminum nitride device is to be subsequently grown on the second substrate layer 104, the second substrate layer 104 should be formed with a composition of (Si0.5-xC0.5-y)Gex+y to yield an in-plane lattice constant of 3.11 Å.
The first substrate layer 102 and the second substrate layer 104 comprise a substrate 110. The second substrate layer 104 has a lattice constant that closely approximates the lattice constant of gallium nitride and other materials formed in the gallium nitride material system. In one embodiment, an optoelectronic device formed using the gallium nitride material system is formed over the substrate 110 and directly over the second substrate layer 104. This is illustrated as a gallium nitride device formed as layer 106 over the second substrate layer 104. A typical device will consist of a number of layers having varying compositions of boron, aluminum, gallium, indium and nitrogen. In this example, the layer 106 of gallium nitride has a lattice constant of 3.19 Å, which closely matches the lattice constant of the second substrate layer 104. The close lattice match between the materials of the layers 104 and 106 allows the gallium nitride material of layer 106 (and subsequent layers which are not shown) to be grown dislocation-free to a thickness that is thicker than if the gallium nitride layer 106 were grown directly on the silicon carbide layer 102. This allows a gallium nitride-based device having high optical quality to be formed over the substrate 110.
The thickness to which the layer 106 can be formed over the second substrate layer 104 before dislocations begin to appear is greater than the thickness to which it could be formed directly over the silicon carbide layer 102. In this manner, a gallium nitride-based device having high optical quality can be formed. The lattice mismatch between the second substrate layer 104 containing about 5%-to about 40% germanium is significantly lower than the lattice mismatch between the first substrate layer 102 and the gallium nitride material of layer 106. The close lattice match between the second substrate layer 104 and the gallium nitride material of layer 106 allows the material of layer 106 to be grown significantly thicker without dislocations forming than if the layer 106 were grown directly over the first substrate layer 102.
A waveguide layer 212 is formed over the active region 210. The waveguide layer 212 is approximately 0.1 um thick and is formed using AlxGa1-xN, where x=0.06. A p-type cladding layer 214 is formed of aluminum gallium nitride over the waveguide layer 212. The cladding layer 214 is approximately 0.4 um thick and is formed using AlxGa1-xN, where x=0.12. A p-type layer 216 of gallium nitride is formed over the cladding layer 214. In one embodiment, the gallium nitride layer 216 is approximately 0.1 um thick. An n-type contact 218 is formed on the substrate 110 and a p-type contact 222 is formed on the p-type layer 216 of gallium nitride.
This disclosure describes the invention in detail using illustrative embodiments. However, it is to be understood that the invention defined by the appended claims is not limited to the precise embodiments described.
