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The primary challenges in implementing a Si based quantum cascade laser are discussed. Intersubband absorption measurements were carried out on a series of modulation doped multiquantum well structures. The spectra were compared to the predictions of a 6 band k.p model, which confirmed the excellent accuracy of the model, and its ability to predict the bandstructures of more complicated cascade structures. A detailed structural analysis demonstrated excellent growth quality, with an interface roughness of < 0.4 nm. Electroluminescence measurements on cascade structures with doped contacts, processed as finger structures and waveguides of various sizes, enabled a quantitative analysis of the active region performance. The upper state lifetime τnr was ∼ 100 fs, leading to a total active region optical gain of ∼ 2 cm−1, a factor of ∼ 10 lower than the estimated total losses due to free carrier absorption. The total emitted power and the linewidth of the intersubband emission saturate above ∼ 6.5 kA/cm2, probably due to misalignment of the injector levels at high biases. The effect of leak currents and interspersed light hole states on the intersubband emission are considered.
We have investigated magneto-transport properties of differently strained Si/ Si1−xGex resonant tunneling devices. The built-in strain was either put in the Si layers, by means of a thick., relaxed Si1−xGex buffer layer, or in the Si1−xGex layers, in which case all Si1−xGex layers were grown below the critical thickness, and a Si1−xGex spacer layer with graded Ge content was used. Magnetic fields parallel to the interface have been employed to probe the in-plane dispersion in the quantum well. This is used to study the effect of band-mixing in the two strain configurations. A field perpendicular to the interface resolves some Landau level splitting. Most strikingly, however, is the similarity in the spectra with the case when the magnetic field is applied parallel to the interfaces. This indicates broadening of the levels, possibly due to scattering, and the importance of 3-dimensional band structure effects.
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