Substituting AlAs for Al0.25Ga0.75As barriers in terahertz (THz) quantum-cascade lasers (QCLs) results in a larger energy separation between the subbands, which reduces the probability for leakage currents through parasitic states and for reabsorption of the laser light. Furthermore, the higher barriers lead to a shift of the quasi-continuum of states to much higher energies. The use of a binary barrier material may also suppress detrimental effects due to the expected composition fluctuations in ternary alloys.
The design of GaAs/AlAs QCLs requires a careful analysis of the impact of the rather high AlAs barriers on the carrier transport since the resonant tunneling is weaker for higher barriers. In order to compensate for this mechanism, the barrier thicknesses have to be rescaled. However, the interface grading requires a stronger reduction than expected for an ideally rectangular potential profile. Therefore, the interface grading is included in our model.
The growth of the very thin AlAs barriers with 2–4 monolayer thickness is rather challenging. The straightforward way to increase the Al shutter opening time for such a thin AlAs layer would be to substantially decrease the AlAs growth rate. However, this would lead to very different V/III flux ratios during the growth of the AlAs and GaAs layers, since the As4 flux cannot be adjusted individually for every single layer. Our trade-off approach comprises nominal growth rates of 0.11 and 0.17 nm/s for AlAs and GaAs, respectively, leading to a minimum Al shutter opening time of 5 s for the thinnest barrier and an overall growth time of about 18 h for the cascades.
Figure 1 shows light-current density-voltage (L-I-V) curves for a 4.75 THz QCL with Al0.25Ga0.75As and for two lasers based on AlAs barriers (with different doping densities) under pulsed operation. While the optical power achieved by the GaAs/AlAs QCLs is similar or even larger than by the GaAs/Al0.25Ga0.75As QCL, the current density and operating voltage is smaller demonstrating that the wall plug efficiency is increased from 3.2x10-4 to 1.2x10-3. As shown in Fig. 2, the lasing frequency of the new laser matches excellently the target frequency of the local oscillator.