Intersubband transitions in strain compensated InxGa1-xAs/AlAs quantum well structures grown on InP substrates

There is considerable interest in developing shorter (< 3 mm) wavelength optical devices based on the intersubband transitions (ISBT) such as ultrafast switchers, modulators and quantum cascade lasers. The choice of the suitable system for short-wavelength ISBT is restricted to the combination of materials that provide an appropriate large conduction band offset. In this letter, we report the optical and structural characterization of the InxGa1-x As/AlAs, x>0.6, quantum well (QW) structures grown on InP substrates. In these structures, an increased In content helps to compensate the larger AlAs tensile strain. Secondly, it provides a smaller InGaAs band gap that results in a shift of the 1st G well subband to lower energies relative to the X minimum in the barrier layers even in very narrow wells.
We have grown InxGa1-xAs/AlAs MQW with InAlAs layers between QW's and superlattice (SL) structures with different well thickness by gas source MBE on semi-insulating InP(001) substrates. Since, the compressively strained InxGa1-xAs layers do not fully compensate the tensile strained AlAs barriers, an In0.55Al0.45As slightly compressive strained buffer layer was inserted. The buffer layer was grown at 490 C and the QW structure at 440 C. The InxGa1-x As wells were Si doped to 2.0x1018cm-3.
The X-ray diffraction pattern and a theoretical fit using a dynamical diffraction theory of the MQW sample with 7ML thick In0.7Ga0.3As and AlAs layers, and 20.0 nm In0.55Al0.45As barriers between the QW's is presented in Fig.1. The data show a very good fit to the model using interfaces with little or no compositional grading.
The transmission spectrum of this sample is shown in Fig.2. The band offsets were determined using the model-solid theory, and the energy-level diagram as calculated within the effective mass approximation of the designed structure is shown in the right inset of Fig.2. There is a relatively large deviation between the calculated and measured ISBT wavelength. Due to the large conduction band offset in this system, ~1.4 eV, the monolayer fluctuations of the well thickness can explain the shift to lower energies of the ISBT for the narrow QW structure compared to the calculated on the assumption of abrupt interfaces as well as its broader ISBT spectrum. In addition, the large In composition and the ability of In atoms to segregate at the surface of the growing InGaAs layer probably leads to excess In at the upper interfaces, although the growth temperature of the QW is decreased to 440 C. This causes a nominal decrease of the In composition in the well at the expense of the formation of an In rich composition in the interface and an eventual intermixing with AlAs, i.e. the formation of an additional InyAl1-yAs interface layer at the bottom side of the barrier. When we incorporate a modified band diagram assuming 1-2 ML In intermixing in the upper interfaces we can calculated an ISBT of 1.91 mm (left insert in Fig.2), which is close to the experimental value. Our interpretation of the intermixing at the interfaces is confirmed by photoluminescence spectra.