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The statistical analysis of thousands of near-field photoluminescence spectra of single ultrathin CdSe layers at 20 K exhibits a strong positive correlation peak around 20 meV energy with a width of 5 meV. Our data are consistent with individual spectra which consist of sets of many pairs of lines. In each pair, the two lines must have comparable strength. We speculate about the origin of these pairs.

We show that energy position and line shape of donor-acceptor-pair luminescence bands in ZnSe:N/ GaAs epilayers depend very sensitively on excitation density and compensation. A continuous development from structureless red-shifted broad to well structured donor-acceptor-pair ͑DAP͒ bands is observed for increasing excitation density. The red shift is explained by the fluctuating potential affecting the bands and impurity levels and is caused by random distribution of charged impurities in highly compensated samples. The shift is reduced when these charge fluctuations are diminished due to an increasing number of impurities being neutralized via light-induced carrier excitation. These effects have not been taken into account in previous work concerning doped II-VI materials; however, they have to be considered when evaluating the frequently used hypothesis of a deep donor in ZnSe:N as an explanation of low-energy broadband DAP emission. The influence of band fluctuations on the behavior of the DAP luminescence and excitation spectra is qualitatively discussed.

Conventional and high-resolution transmission electron microscopy (TEM) was applied to a study of the Cd distribution and structure of defects in single and multiple CdSe layers in a ZnSe matrix. The samples containing nominal CdSe layer thicknesses t n between 1.7 and 3.5 monolayers (ML) were grown by molecular beam and atomic layer epitaxy under different conditions. In all samples ternary CdZnSe wetting layers with Cd-rich regions with sizes of less than 10 nm (small islands: SIs) and a density of $10 11 cm --2 are observed. The Cd concentration in the wetting layer and the SIs increase with t n . In addition, regions with sizes of 20-30 nm (large islands: LIs) and Cd concentrations >40% occur in specimens with t n > 2.5 ML. In the vicinity of the LIs stacking faults are preferably generated leading to a "coffee-bean" contrast in plan-view TEM images. The multilayer structures with t n % 2.5 ML display a predominant vertical correlation of the SIs at a ZnSe spacer thickness of 12 ML. IntroductionA considerable number of studies about CdSe/ZnSe quantum dot (QD) structures were recently carried out which are motivated by the perspective of incorporating QD structures in laser diodes for the blue-green spectral range. The coexistence of two size classes of self-organized islands was previously observed [1-3]. Cd-rich regions with a size of less than 10 nm (small islands: SIs) appear even at nominal thicknesses t n < 1 monolayer (ML) which are embedded in a broadened wetting layer (WL) [2][3][4][5]. Large three-dimensional islands (LIs) with sizes of significantly above 10 nm occur at larger t n in the range of 2.1-3 ML [1-3] whose origin can be associated with a transition from the two-(2D) to the three-dimensional (3D) growth mode. Krestnikov et al. [5] investigated CdSe/ZnSe superlattices with t n < 1 ML and varying thicknesses of the ZnSe spacer and found a vertical correlation of the Cd-rich inclusions for thin (1.5 nm) ZnSe spacers. In the present paper we present a transmission electron microscopy (TEM) study of the structural and chemical properties of single CdSe/ZnSe structures with different t n and multiple CdSe/ZnSe structures with t n % 2.5 ML and various ZnSe spacers.

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Transmission electron microscopy investigation of structural properties of self-assembled CdSe/ZnSe quantum dots Appl.The origin of the ''coffee-bean'' strain contrast is studied, that is observed in the plan-view transmission electron microscopy ͑TEM͒ images of CdSe/ZnSe quantum dot structures. The samples were grown by two different methods: standard molecular-beam epitaxy at 350°C and atomic layer epitaxy at 230°C with annealing at 340°C after the CdSe deposition. The nominal CdSe thickness was above 3 ML. In situ reflection high energy electron diffraction during the growth or during the annealing shows the transition from the two-͑2D͒ into the three-dimensional ͑3D͒ surface morphology for both samples. The coffee-bean contrast is usually assigned to three-dimensional islands which are generated after the morphological 2D/3D transition. It is found that the coffee-bean contrast in plan-view TEM images is alternatively associated with pairs of stacking faults on ͕111͖ lattice planes which are inclined against each other. The stacking faults, which are bound by Shockley partial dislocations, are preferably generated in the vicinity of the Cd-rich regions ͑large islands͒ of the CdZnSe layer where Cd concentrations of more than 40% are found.

We report a remarkably low stacking-fault density in ZnSe epilayers directly grown on commercial epi-ready GaAs (001) substrates without GaAs buffer layer growth. It is found that proper pregrowth treatments on epi-ready GaAs (001) substrates to obtain clean surfaces are crucial for two-dimensional layer-by-layer growth and suppression of stacking fault generation. Chemical etching using a NH4OH-based solution is found to reduce not only the thickness of the oxide layers but also the ratio of Ga2O3 to As2O3 to about half of that before etching. A clean GaAs (001) surface characterized by a (4×1) reconstruction in the present case is obtained after thermal cleaning followed by Zn pre-exposure. Reflection high-energy electron diffraction intensity oscillations with more than 50 periods are observed even from the very beginning of ZnSe growth on GaAs substrates cleaned as such. The stacking fault density in such a ZnSe layer is in the low-105 cm−2 range.

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