InAs monolayer (ML) plays a critical role in controlling the density and size of dots in QDs heterostructures grown by S-K mode using MBE. The density of QDs increses very sharply up to an InAs deposition of 2.0 ML and gets saturated over a 3.0 ML thickness. It is kown that for single layer QDs (SQD) the wavelength of emission peak associated with QDs gets extended as the thickness of InAs layer increases from 2.0 ML to 3.5 ML, with a considerable reduction in the full-width-at-half-maximum (FWHM), indicating formation of larger dots with homogenius size.

In this paper, we have shown that by using bi-layer QDs (BQD), which are grown having two layers of InAs coverage with a spacer layer of GaAs inserted in between them, the emission wavelength can be extended further, with a more uniform distribution of dots as compared to SQD. We have comapered structural and optical properties of an SQD with BQD samples, with higher monolayer coverage, and proved the superioty of BQD sample over the SQD sample in terms of optical and structural quality.

From AFM images we have found that the dots in the BQD show greater homogeniety, with lower size variation, as compared to SQD, which shows a bimodal distribution. The dot density as estimated from AFM are 1.5x10¹⁰/cm² and 1.4x10¹⁰/cm² for the SQD and BQD samples respectively. PL experiment is done at 8K, under an excitation of 40mW. The ground state (GS) peak of SQD is found at 1157 nm with an FWHM of 34.5meV while for the BQD, the GS peak is at 1229 nm with an FWHM 16.7 meV. The greater homogeneity in BQD sample is indiacted by lower line width of emission peak. It is attributable to the uniform formation of dots in top layer under the strained field produced by the seed layer, being a separation of only 8.5 nm spacer thickness between two layers. The red shift of emission peak for BQD sample indicates the growth of larger dots in comparison to SQD sample.

DST, India and SPM,IITB is gratefully acknowledge for their support.

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