SiGe technology
B
and-gap engineering is one of the most important guidelines to design new semiconductor alloys and devices. For the last 60 years, two important semiconductor alloy engineering models were developed for band-gap engineering; the cubic crystal structure model shown in Figure 1-(a) and hexagonal crystal structure model shown in Figure 1-(b). The first cubic crystal model includes group IV semiconductors (Si,Ge,C) of diamond structure, and group III-V (GaAs, InP, etc.) and group II-VI (ZnSe, CdTe, etc.) semiconductors of cubic zinc-blende structure. The second hexagonal crystal model includes III-Nitride semiconductors (GaN, AlN, InGaN, etc.) and hexagonal SiC semiconductors.
Our team at NASA Langley Research Center proposed and proved that a third alloy engineering model - “rhombohedral-trigonal crystal model” - can be established between the cubic one and hexagonal one. A simple diagram of rhombohedral-trigonal crystal model is shown in Figure 1-(c). In details of the new model, a general cubic crystal is not only a special case of tetragonal crystal but also a special case of a rhombohedral crystal with inter-planar angle of 90°. When a cubic crystal is strained in the [111] direction, it becomes a rhombohedron. Thus, a cubic crystal belongs to a rhombohedral crystal group.
Additional mathematical transformation equation transforms any rhombohedral crystal into a trigonal crystal in hexagonal frame. Therefore, cubic crystals belong to a more general trigonal crystal group and the epitaxy between cubic crystals and trigonal crystals can be established not as an accidental coincidence-lattice- matching but as a fundamental crystal symmetry relation. Figure 1-(d) shows such an example of rhombohedrally aligned cubic SiGe on trigonal c-plane sapphire. The problem of this epitaxy is that two crystal structures which are twin to each other can be formed as shown in Figure 1-(e), the top view. This twin defect was a major problem in the rhombohedral epitaxy and has hindered further applications so far.
However, we found that optimized growth under new X- ray diffraction (XRD) characterization can eliminate twin defect and form single crystalline rhombohedral SiGe layer on c-plane sapphire in one of the crystal alignment of Figure 1-(e). This is because threefold symmetry of a trigonal crystal prefers one rhombohedrally aligned cubic crystal to the other. Thus, a symmetry breaking occurs between two cubic crystals that are rotated by 60° from each other, i.e. one cubic crystal becomes dominant and the other cubic crystal diminishes.
The discovery of super-hetero-epitaxy growth technology for rhombohedral single crystalline SiGe on c-plane sapphire was confirmed by the NASA-invented new XRD methods: (1) Total defect density measurement and (2) Spatial wafer mapping method (see further reading,
Figure 1. Crystal structure of (a) cubic zinc-blende or diamond structure, (b) hexagonal Wurtzite structure, (c) crystal symmetry group relation, (d) Rhombohedrally aligned SiGe on c-plane Sapphire, (e) two possible alignments of SiGe on trigonal c-plane Sapphire, twin to each other
Figure 2. Innovative patented XRD methods characterize integral density and spatial distribution of twin defects
patents pending 2, 4, and 5). Figure 2 shows the characterization results by XRD. Innovative XRD wafer mapping method shows spatial distribution of major single crystalline SiGe (99.8%) (Left image) and twin defect SiGe (0.1%) (Right image) that exist on the same sapphire wafer clipped by three plastic jaws outside. Twin crystal defect is reduced to below 0.1% and it exists only at the edge of a wafer. The successful development of rhombohedrally aligned SiGe has also led the inventors to construct a new hybrid bandgap engineering diagram with transformed lattice constants. In conventional cubic bandgap engineering diagram, the distance of [100] vector, i.e. lattice constant of a cube is used as coincidence lattice distance. On the other hand, conventional hexagonal bandgap engineering diagram uses the distance of basal plane basis vector as
April/May 2010
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