Resonant Gain in ZnSe Structures with Stacked CdSe Islands Grown in Stranski‐Krastanov Mode

The optical gain of CdSe/ZnSe structures containing stacks of islands grown in the Stranski-Krastanow mode is studied. The islands have a lateral size of ~16 nm and a small height-to-width ratio. Cd contents of ~70 % were estimated from high-resolution transmission electron microscopy investigations. The radiative recombination of excitons localised in the islands lead to a broad emission band around 2.4 eV. At low temperatures resonant gain up to 300 cm was found generated by localised transitions. Introduction. Wide bandgap II-VI quantum dot structures based on self-organised island growth were achieved by several growth modes [1] resulting either in the formation of small 2D-like islands [2] or in islands formed in a three-dimensional (3D) growth process [3,4,5]. Zero-dimensional excitons localised in such islands enable resonant gain. Therefore, efficient excitonic waveguiding [2] can be applied to reduce the threshold density of lasers devices. For structures containing small islands, labelled type A, zero-phonon gain [e.g.,6,7] and optical pumped lasing at room temperature [8,9,10] were achieved. Nevertheless, the limited thermal stability of carrier localisation in type A islands prevents room temperature (RT) operation. Recent investigations on CdSe/ZnSe structures [11] have demonstrated that at room temperature excitons are evaporated from type A islands whereas thermal stability was found for larger islands, grown in the Stranski-Krastanow (SK) mode (labelled type B). In order to take advantage of the better thermal stability in type B islands, we have studied the resonant gain in structures containing such islands. Experimental. The investigated structures were grown by molecular beam epitaxy (MBE) and consist of a single layer or a 10 fold stacks of nominally 2.8 ML thick CdSe coverage deposited at 340 oC. The CdSe layers in the stacks were separated by 12 nm ZnSe spacers. All structures were cladded by 45 nm thick ZnSe bufferand cap layers. To study the 1 on leave from Ioffe-Institute, Polytekhnicheskaya 26, St. Petersburg, 194021, Russia physica status solidi –2– M. Strasburg et al. conditions for the formation of type A islands which have a Cd content below 40 % and those of type B islands which are described in detail in previous publications [5,11], samples were grown both, on exactly (001)-oriented GaAs (exactly oriented) substrates and on substrates which were tilted by an angle of 6 o towards the direction (tilted). The growth on tilted surfaces is believed to enable the step-flow growth mode, and thus to suppress the formation of type A islands. The formation of 3D islands was confirmed by in-situ monitoring of the RHEED pattern. More details of the growth are given in Ref. 12. The samples were studied by transmission electron microscopy (TEM) in plan-view and cross-section geometry using a Philips CM 200 FEG/ST electron microscope with an electron energy of 200 keV and a Scherzer resolution of 0.24 nm. For the optical investigations the samples were mounted in a He-flow cryostat providing temperatures between 4 K and 300 K. The photoluminescence (PL) was excited by the 325 nm line of a cw He-Cd laser. Gain studies were performed in edge geometry using the variable-stripe-length method [13]. These measurements used a pulsed dye laser with ∼20 ns pulse duration pumped by an excimer laser at an excitation energy of 2.85 eV. Luminescence was detected by a photomultiplier attached to a 0.85 m double monochromator. Gain spectra were recorded at 8 K. Results and Discussion. A typical cross-section TEM image of the sample with stacked SK islands (type B) grown on tilted substrate is depicted in Fig. 1. A relative large number of type B islands is revealed by regions with a bright contrast inside the islands. This contrast indicates a high Cd concentration which was estimated to ~70 % [12]. Type B islands having an average lateral size of 16 nm are laterally surrounded by an ~3 nm thick wetting layer (WL) with a significantly lower Cd concentration. The height of the islands exceeds that of the WL only slightly. In some places, the island arrangement is laterally shifted in adjacent