Abstract
We have investigated the emission frequency of terahertz (THz) quantum-cascade lasers (QCLs) as a function of the location on the wafer. The frequency varies due to an inhomogeneous growth rate across the wafer. For three wafers based on GaAs/AlAs heterostructures for lasers with target frequencies of 3.36 and 3.92 THz, we observed a blue shift of the emission frequency from the center to the edge of the wafer. This blue shift is attributed to a decrease of the period length of the QCLs, which can be determined with spectroscopic techniques. The location-dependent period length is used to calculate a position-dependent frequency of the gain maximum for the active region. The correlation of the calculated frequencies with the emission frequencies of lasers fabricated from different locations on the wafer allows us to establish an effective method for the fabrication of THz QCLs emitting at a particular target frequency.
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