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Left: Atomic force microscope (AFM) scan of SiGe fabricated by the conventional technique. Root mean square (RMS) roughness is 13 nm. The crosshatch is a sign of severe dislocation formation. Right: AFM scan of our SiGe substrate. RMS roughness = 0.27 nm. |
B. Tanto, D. E. Savage, M. G. Lagally (DMR #0520527)
To make modulation-doped 2-dimensional electron-gas structures for applications in quantum electronics, the requirements on the quality and strain state of the materials for the Si/SiGe heterojunctions are extremely stringent. Defect-free strained silicon and silicon/germanium alloy will be ideal. Such perfect materials currently do not exist. We have developed a new process for fabricating defect-free strained-Si-based materials. While strained Si is well established, the material is far from perfect, containing dislocations and non-uniform strain. Electron scattering reduces performance of modulation-doped strained-Si quantum wells. Interface roughness and non-uniform strain contribute to electron scattering, and hence must be minimized. These new materials will make possible fundamentally new experiments in quantum electronics. Improving the Si material quality may also improve device uniformity and performance of high-mobility electron channel devices.
We make thin tri-layer films of Si/SiGe/Si deposited epitaxially on silicon-on-insulator. We free the membrane by etching the oxide with HF. The membrane is further exposed to KOH, a preferential Si etch, leaving behind a freestanding fully-relaxed SiGe film, which is then bonded to a new host substrate. Specifically, we use molecular beam epitaxy (MBE) to grow 20 nm of compressively strained SiGe with 33% Ge concentration well below its kinetic critical thickness on a Si substrate, thus obtaining defect-free SiGe. The SiGe is allowed to relax fully after the etch process. The figure shows a comparison of the surface of conventionally fabricated SiGe substrates and our new SiGe substrates.
Because our new substrates are smooth and defect free, they are outstanding as a growth substrate for strained silicon. We have demonstrated growth of smooth (0.26 nm root mean square roughness) 1.3% strained Si on our SiGe substrate. These are to our knowledge the first examples of highly strained dislocation-free Si. Si strained to this extent will show a reduction in its band gap by about 20%, i.e., more than 200meV.
