Abstract
A characteristic feature of quantum cascade lasers is their unipolar carrier transport. We exploit this feature and realize nominally symmetric active regions for terahertz quantum cascade lasers, which should yield equal performance with either bias polarity. However, symmetric devices exhibit a strongly bias polarity dependent performance due to growth direction asymmetries, making them an ideal tool to study the related scattering mechanisms. In the case of an InGaAs/GaAsSb heterostructure, the pronounced interface asymmetry leads to a significantly better performance with negative bias polarity and can even lead to unidirectionally working devices, although the nominal band structure is symmetric. The results are a direct experimental proof that interface roughness scattering has a major impact on transport/lasing performance.
Highlights
The emission wavelength of quantum cascade lasers (QCLs), currently ranging from the midinfrared to the terahertz (THz) spectral region (3–300 μm) [1,2], can be engineered by designing subband levels in a semiconductor heterostructure
An early work on mid-infrared QCLs was focused on dual wavelength operation with either bias polarity and a symmetric active region was merely demonstrated as a proof of principle [3]
The results prove that rough interfaces, even if they are only present on one side of the barrier, play a major role in the transport and performance of THz QCLs, confirming theoretical predictions of non-equilibrium Green’s function calculations by Kubis et al
Summary
The emission wavelength of quantum cascade lasers (QCLs), currently ranging from the midinfrared to the terahertz (THz) spectral region (3–300 μm) [1,2], can be engineered by designing subband levels in a semiconductor heterostructure. This unique feature is accomplished by band structure engineering. The experimental challenge lies in the direct observation of the influence of a certain scattering mechanism in QCLs. The experimental challenge lies in the direct observation of the influence of a certain scattering mechanism in QCLs For this purpose we exploit the unipolar carrier transport feature of QCLs. Combined with a high degree of freedom due to band structure engineering, one can design bidirectional dual wavelength or nominally symmetric active regions. An early work on mid-infrared QCLs was focused on dual wavelength operation with either bias polarity and a symmetric active region was merely demonstrated as a proof of principle [3]
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