SANDiE Partner P14

Consiglio Nazionale delle Ricerche (CNR-IMEM, Parma), Italy

It was previously shown by CNR-IMEM (P14) that QD strain engineering can be used to redshift the emission of InAs/InGaAs QD structures grown on GaAs substrates: in structures with metamorphic InGaAs buffers the partial lattice-relaxation of buffers and, then, the QD strain is determined by both the composition of confining layers (CL) and the thickness of the lower one; the availability of two degrees-of-freedom allows the control of both the energy-gap of the QD material – that affects the emission wavelength – and the band-discontinuities confining carriers into the QDs – that also determine the activation energy of thermal quenching of luminescence emission and, then, the emission efficiency. By means of experimental results and model calculations it was shown that the use of high-In-content InGaAs layers as confining layers to redshift the emission wavelength inevitably results in the decrease in band discontinuities and, then, in emission efficiency. Therefore, it has been studied the effect of additional InAlAs barriers embedding InAs QDs to increase the carrier confinement into QDs and, hence, the RT emission efficiency. By concomitantly using QD strain engineering and barrier enhancing, the blueshift of emission ensuing from the use of higher barriers can be compensated by the decrease of the energy-gap of QD material.

By optimizing the design of structures (composition and thickness of confining layers and composition of additional barriers) photoluminescence emission wavelengths as long as 1.59 µm have been obtained at RT from metamorphic InAs/InGaAs QD structures with additional InAlAs barriers, grown on GaAs substrates; this result, obtained by using excitation power densities as low as 5 W/cm2 has been rarely reported so far and is of wide interest for photonic applications of QD nanostructures. In order to further increase the RT emission efficiency, InAs/GaAs structures have been studied by AFM, space-charge techniques and TEM; the formation of ripened large-sized QDs has been observed for coverages larger that 2.4 ML; such islands coexist with small-sized coherent QDs. Large-sized QDs progressively relax as the InAs coverage increases and, consequently, the PL integrated intensity significantly decreases and deep electronic levels, that can be related to extended structural defects, show up. Cooperations on different aspects of the research have been established with P02 (Leuven), P08 (Sheffield), P16 (Valencia), P17 (Cádiz) and P29 (Lancaster).

By means of a number of X-ray techniques and TEM defects and phase tranformation has been studied in MBE-grown MgS layers in MgS/ZnSe/GaAs structures of interest for MgS/ZnSe QDs. The partial nucleation of MgS with rock-salt structure, unexpectedly coexisting with the stable zinc-blende MgS phase, is evidenced and correlated to the presence of crystallographic defects (stacking faults) at the bottom interface (in cooperation with a P10 (Edinburgh)).

Homepage

last update: 28 February 2019, A. Weber