The emission wavelength of multilayer InAs/GaAs QDs using GaAs capping is typically around 1.1 μm, which is short compared to the 1.3-1.55 μm used in communication devices. In this experiment we report a new method for tuning 1.33 μm emission by increasing the size of QDs while maintaining a low defect density and fast grow rate by adding InAlGaAs quaternary capping (same lattice constant with GaAs) beside GaAs layer. The MBE grown heterostructures consisted of 10 layers of 2.7 ML InAs QDs with various thickness of capping. sample A and B have GaAs capping of thickness of 65Å and 70Å, while the quaternary capping layer thickness is increased from 20Å to 30Å. Sample C has 30Å of InAlGaAs capping and 150Å of GaAs capping. The samples were characterized by AFM, TEM and Photoluminescence spectroscopy. The TEM micrographs of A and B reveal that the number of complete Qd stacks has decreased and defect density has increased in B. Threading dislocations which reach the surface have caused some large dots (dia ~ 100 nm) to form on the surface in both samples, as seen in the AFM image. The Pl spectra illustrate the ground state emission peak at 1300 nm in B is red shifted by about 65 nm as compared to A, an indication of large dot formation. The QDs are enlarged because of strain driven migration of Indium atoms into the dots from thicker quaternary InAlGaAs capping in B compared to sample A. The PL spectrum shows that the emission peaks for both B and C are formed at same wavelength, indicative of dots of equal size. The integrated PL intensity in C is much higher which is because of the absence of defects. sample C can better accommodate the increased strain arising from larger dots because of a thicker GaAs capping layer. Thus to achieve the desired red shift, an InAlGaAs quaternary capping layer can be added to increase the effective size of QDs and thick GaAs layer maintaining defect free heterostructure. DST India and SPM facility IIT Bombay are being acknowledged.   

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