Intersubband transitions in strained indium gallium arsenide quantum wells for multi-color infrared detector applications

Intersubband transitions in In_xGa_{1− x}As/AlGaAs multiple quantum wells (MQWs) grown by molecular beam epitaxy (MBE) were studied. The conduction band offset for this material system is larger than that of the well-known GaAs/AlGaAs system, thus making it possible to design, grow and fabricate quantum well infrared photodetectors operational in the 5–8 μm and 10–14 μm spectral regions with minimal dark current. In_xGa _1−xAs/AlGaAs MQWs were grown by MBE with indium compositions ranging from x = 0.10 to 0.15 verified by in situ RHEED oscillations and high-resolution X-ray diffraction. Band-to-band transitions were verified by photoluminescence measurements, and intersubband transitions were measured using Fourier transform infrared (FTIR) spectroscopy in both the Brewster's angle and waveguide configuration. Due to the high strain and introduction of dislocations associated with the high indium content, wells with indium compositions above ∼12% did not result in intersubband transitions at silicon doping levels of 2 × 10 18 cm−3. New structures were grown, with a thick linear graded In_xGa_{1− x}As buffer below the MQW structures to reduce the strain and resulting dislocations. Intersubband transitions were measured in In _xGa_1−xAs wells with indium compositions of x = .20 when grown on top of the linear graded buffer (LGB). Three-color device structures consisting of In_xGa_1−xAs triple-coupled MQWs were grown with and without the LGB. FTIR measurements revealed that without the LGB, intersubband transitions were not present in the three-color structure. However, with the LGB intersubband transitions were measured. Only one intersubband peak was observed in the three-color structures in the Brewster angle configuration—possibly due to nonuniformity in the sample growth. In the waveguide configuration, an additional higher energy peak was observed which other groups have attributed to multiple internal reflections off the many layers in the structure. One three-color structure with an LGB layer and 21.4% indium showed multiple peaks in the waveguide configuration, but no peaks in the Brewster angle configuration. Due to the complex nature of the waveguide, it is difficult to compare the measured peaks with the expected values. However, this sample showed the most promising results in terms of multi-color detection behavior.