Motorola grows GaAs on silicon

Though gallium arsenide (GaAs) and other III-V compounds offer better carrier mobility and more useful optoelectronic properties than silicon, silicon dominates the global semiconductor market. Much of silicon's success can be attributed to superior manufacturability. SiO₂ offers a robust passivating layer, silicon is a common and relatively inexpensive material, and silicon wafers are less fragile.

As high frequency and optoelectronic devices make their way into consumer applications like cell phones and CD players, manufacturers of these devices would like to realize greater economies of scale. Because III-V wafers are so fragile, scaling up to the 8" and 12" sizes used in silicon IC manufacturing has been difficult.

Motorola, in a series of presentations over the last several weeks, has announced a method for growing GaAs on silicon wafers. The resulting material meets dislocation and mobility specifications for bulk GaAs. It is a suitable substrate for field effect transistors, and is being evaluated for LEDs and lasers.

William Ooms, the Motorola Labs director of materials, device, and energy research, explained that the GaAs lattice constant (5.65 Å) is about 4% larger than silicon's (5.43 Å). This difference makes it difficult to grow high quality GaAs directly on silicon. Instead, the Motorola researchers placed a thin layer (about 50 Å) of strontium titanate (SrTiO₃) between the two semiconductors. Strontium titanate's lattice constant falls roughly halfway between GaAs and Si. Moreover, an amorphous layer (10-20 Å thick) forms between Si and SrTiO₃. This layer absorbs the lattice mismatch strain between the two materials, allowing the crystalline SrTiO₃ to form a normal lattice without distortion from the underlying Si. Because the SrTiO₃ layer is very thin, the amorphous interlayer also absorbs the mismatch strain between the SrTiO₃ and GaAs layers.

Motorola deposits both SrTiO₃ and GaAs by molecular beam epitaxy (MBE). Though MBE is expensive, it is still more economical than bulk GaAs wafers of comparable size. Epitaxial wafer supplier IQE is working with Motorola to commercialize the process, Ooms said. Researchers are also studying the feasibility of a similar process based on chemical vapor deposition (CVD).

Motorola has tested prototype RF circuits using the substrates and plans to qualify them for production beginning next month. Beyond the immediate application--cost reduction for GaAs circuits--the material has the potential for seamless integration of Si and GaAs devices. Ooms said that Motorola plans to build GaAs devices in designated areas of CMOS wafers soon. The company is also investigating a similar approach to integration of other III-V materials.

Motorola grows GaAs on silicon

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