Frank Tinjod

II–VI quantum dot formation induced by surface energy change of a strained layer

A method for growing self-assembled II–VI quantum dots (QDs) is demonstrated: A highly strained CdTe layer, grown onto Zn(Mg)Te, is covered with an amorphous Te layer which is then desorbed. This induces QD formation, observed as an abrupt change of both the reflection high-energy electron diffraction pattern and the surface morphology studied by atomic force microscopy in an ultrahigh vacuum. The dots are also characterized after capping by microphotoluminescence. This morphology transition, which occurs after and not during the growth, can be understood in terms of variation of the surface energy in presence of the group-VI element, which compensates for the natural trend toward plastic relaxation in II–VI compounds. This method shows the strong influence of the surface energy (and not just the lattice mismatch) in inducing the formation of coherent islands for mismatched systems having a low dislocation formation energy such as CdTe/ZnTe and CdSe/ZnSe.

Correlated photon emission from a single II–VI quantum dot

We report correlation and cross-correlation measurements of photons emitted under continuous wave excitation by a single II–VI quantum dot (QD) grown by molecular-beam epitaxy. A standard technique of microphotoluminescence combined with an ultrafast photon correlation setup allowed us to see an antibunching effect on photons emitted by excitons recombining in a single CdTe∕ZnTe QD, as well as cross correlation within the biexciton (X2)-exciton (X) radiative cascade from the same dot. Fast microchannel plate photomultipliers and a time-correlated single photon module gave us an overall temporal resolution of 140ps better than the typical exciton lifetime in II–VI QDs of about 250ps.

CdTe/Zn1−xMgxTe self-assembled quantum dots: Towards room temperature emission

We report the dependence of the growth and the optical properties of self-assembled CdTe/Zn1−xMgxTe quantum dots on the barrier Mg content x (0⩽x⩽0.3). Due to the decrease of the lattice mismatch between CdTe and Zn1−xMgxTe with increasing x, we use a technique for inducing dot formation, based on efficient reduction of the surface energy by deposition of amorphous Te, which is then desorbed. Mg incorporation in the barriers leads to a better heavy-hole confinement along the growth axis, which is manifested in photoluminescence (PL) studies by both an extension of the radiative regime temperature range (up to 150 K for 30% Mg) and a strong increase of the activation energy for the nonradiative recombination. However, the in-plane confinement is less enhanced, which allows observation of interdot carrier transfer with increasing temperature, as evidenced directly by the analysis of PL intensities for different single dots. Our temperature-dependent data (time-resolved and microphotoluminescence) suggest th...

Thermal escape of carriers out of CdTe/ZnMgTe nanostructures

We report on time-resolved photoluminescence (PL) measurements up to room temperature of CdTe/Zn1−xMgxTe narrow quantum wells (QWs) and quantum dots (QDs). Whereas the low-temperature dependence of the PL decay time τ is characteristic of systems dimensionality, the Arrhenius plot of its high-temperature dependence directly gives the thermal activation energy of the loss channel limiting the emission in these nanostructures. By varying the Mg-content of the barriers, we unambiguously identified the unipolar escape of the less confined carriers, namely heavy holes, to be the main non-radiative mechanism. With increasing Mg-barrier composition, both CdTe QWs and QDs show a radiative regime extended to higher temperatures together with a non-radiative regime exhibiting a higher activation energy.