Optimization of strain-coupled InAs QD layers in P-i-P infrared photodetector heterostructures

Strain in the heterostructure plays a vital role in the characteristics of Quantum Dot (QD) based optoelectronic devices. Optimization of the number of dot layers to be strain-coupled is analyzed here to attain QD infrared photodetectors with higher efficiency. Heterostructures are grown in a molecular beam epitaxy (MBE) system with two (Bi), three (Tri), five (Penta) and seven (Hepta) strain-coupled QD layers, to observe the variation in the optical properties. The effect of thin In0.15Ga0.85As Strain Reducing Layer (SRL) over these coupled structures is also analyzed. Photoluminescence (PL) and Photoluminescence Excitation (PLE) spectroscopy are carried out on the grown structures. Low-temperature Power Dependent PL and PLE revealed the discrete energy states in the dots. The ground state (GS) peaks are found at 1.16 eV, 1.18 eV, 1.195 eV, and 1.194 eV for Bi-, Tri-, Penta-, and Hepta-layer structures. The corresponding peaks redshifted to 1.12 eV, 1.14 eV, 1.154 eV, and 1.152 eV, with the incorporation of 2 nm SRL. It is observed from the PLE results that the excited state peaks of Bi-to-Heptalayer structures are 68 meV, 70 meV, 74 meV, and 72 meV away from the GS peak. However, the differences obtained for the samples with In_0.15Ga_0.85As SRL are 59 meV, 66 meV, 68 meV, and 70 meV. It is seen that the GS PL peaks of Penta-layer samples with both kinds of structures have the highest intensity. The study shows the importance of strain-coupling and provides an optimum QD heterostructure for better device performance.