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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.
One of the major challenges during recent years was to achieve the compatibility of III-V semiconductor epitaxy on silicon substrates to combine opto-electronics with high speed circuit technology. However, the growth of high quality epitaxial GaAs on Si is not straightforward due to the intrinsic differences in lattice parameters and thermal expansion coefficients of the two materials. Moreover, antiphase boundaries (APBs) appear that are disadvantageous for the fabrication of light emitting devices. Recently the successful fabrication of high quality germanium layers on exact (001) Si by chemical vapor deposition (CVD) was reported. Due to the germanium seed layer the lattice parameter is matched to the one of GaAs providing for excellent conditions for the subsequent GaAs growth. We have studied the material morphology of GaAs grown on Ge/Si PS using atomic layer epitaxy (ALE) at the interface between Ge and GaAs. We present results on the reduction of APBs and dislocation density on (001) Ge/Si PS when ALE is applied. The ALE allows the reduction of the residual dislocation density in the GaAs layers to 105 cm−2 (one order of magnitude as compared to the dislocation density of the Ge/Si PS). The optical properties are improved (ie. increased photoluminescence intensity). Using ALE, light emitting diodes based on strained InGaAs/GaAs quantum well as well as of In(Ga)As quantum dots on an exactly oriented (001) Ge/Si pseudo-substrate were fabricated and characterized.
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