Ultrathin silicon-on-insulator, composed of a crystalline sheet of silicon bounded by native oxide and a buried oxide layer, is extremely resistive because of charge trapping at the interfaces between the sheet of silicon and the oxide. After chemical modification of the top surface with hydrofluoric acid (HF), the sheet resistance drops to values below what is expected based on bulk doping alone. We explain this behavior in terms of surface-induced band structure changes combined with the effective isolation from bulk properties created by crystal thinness.

We demonstrate the feasibility of fabricating heterojunctions of semiconductors with high mismatches in lattice constant and coefficient of thermal expansion by employing nanomembrane bonding. We investigate the structure of and electrical transport across the interface of a Si/Ge bilayer formed by direct, low-temperature hydrophobic bonding of a 200 nm thick monocrystalline Si(001) membrane to a bulk Ge(001) wafer. The membrane bond has an extremely high quality, with an interfacial region of ∼1 nm. No fracture or delamination is observed for temperature changes greater than 350 °C, despite the approximately 2:1 ratio of thermal-expansion coefficients. Both the Si and the Ge maintain a high degree of crystallinity. The junction is highly conductive. The nonlinear transport behavior is fit with a tunneling model, and the bonding behavior is explained with nanomembrane mechanics.

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Abstract. The dependences of the 294 and 10 K mobility μ and volume carrier concentration n on thickness (d ¼ 25 to 147 nm) are examined in aluminum-doped zinc oxide (AZO). Two AZO layers are grown at each thickness, one with and one without a 20-nm-thick ZnON buffer layer. Plots of the 10 K sheet concentration n s versus d for buffered (B) and unbuffered (UB) samples give straight lines of similar slope, n ¼ 8.36 × 10 20 and 8.32 × 10 20 cm −3 , but different x -axis intercepts, δd ¼ −4 and þ13 nm, respectively. Plots of n s versus d at 294 K produce substantially the same results. Plots of μ versus d can be well fitted with the equation μðd Þ ¼ μð∞Þ∕½1 þ d à ∕ðd − δdÞ, where d à is the thickness for which μð∞Þ is reduced by a factor 2. For the B and UB samples, dà ¼ 7 and 23 nm, respectively, showing the efficacy of the ZnON buffer. Finally, from n and μð∞Þ we can use degenerate electron scattering theory to calculate bulk donor and acceptor concentrations of 1.23 × 10 21 cm −3 and 1.95 × 10 20 cm −3 , respectively, and Drude theory to predict a plasmonic resonance at 1.34 μm. The latter is confirmed by reflectance measurements.

SiGe/Si quantum wells are of great interest for the development of Group-IV THz quantum cascade lasers. The main advantage of Group-IV over III-V materials such as GaAs is that, in the former, polar phonon scattering, which significantly diminishes the efficiency of intersubband light emission, is absent. However, for SiGe/Si multiple-quantum-well structures grown on bulk Si, the lattice mismatch between Si and Ge limits the critical thickness for dislocation formation and thus the number of periods that can be grown. Similarly, the use of composition-graded SiGe films as a lattice-matched substrate leads to the transfer of dislocations from the graded buffer substrate into the quantum wells, with a consequent decrease in light emission efficiency. Here we instead employ nanomembrane strain engineering to fabricate dislocation-free strain relaxed substrates, with lattice constants that match the average lattice constants of the quantum wells. This procedure allows for the growth of many periods with excellent structural properties. The samples in this work were grown by low-pressure chemical vapor deposition and characterized via high-resolution X-ray diffraction and far-infrared transmission spectroscopy, showing narrow intersubband absorption features indicative of high crystalline quality.