Unstrained vs. Strained Layer Epitaxy: Thick Ge Layers and Ge/Si Superlattices on Si(100)

Abstract

Smooth unstrained Ge layers (up to 1 μm) have been grown on Si(100) by using low substrate temperatures T_s (≤ 420 °C) for the first few 100 Å of growth, thus suppressing the onset of 3-dimensional growth usually accompanying the generation of misfit dislocations. By increasing T_s up to 550 °C during growth the layers can be made to exhibit excellent crystallinity as evidenced by an RBS minimum channeling yield χ_min of 3.5%. By contrast to these thick Ge layers high quality strained-layer Si/Ge superlattices can only be fabricated by keeping the thickness of the individual Ge layers below their critical thickness (≈ 6 monolayers). In addition the total thickness has to be kept below the critical thickness of the corresponding Si_1-xGe_x alloy. We have grown strained-layer Si/Ge superlattices with periods exceeding 25 Å and with a Ge thickness below 6 monolayers. Crystalline and interface quality have been confirmed by RBS and channeling (χ_min ≈ 5%) and by X-ray diffraction, respectively. The superlattices have also been examined by means of Raman scattering. Up to three orders of folded acoustic phonons have been observed. Furthermore, interface and Ge-modes are shifted due to the strain in the Ge layers.