Metastable interstitials in CdSe and CdS crystals

An "anomalous" defect drift in external electric field, namely, transport of acceptorlike centres from the anode to the cathode, has been observed in CdS:Cu, CdS:Ag and nominally undoped CdSe crystals at 350-700 K. The effect is accounted for by transformation of acceptors into donors under heating. The donors are metastable centres that do not display themselves in the equilibrium state and can be revealed only by drift in electric field. The acceptors are shown to be substitutional impurity atoms, acceptor-to-donor transformation occurring due to transition of these atoms from lattice sites to interstitials. Under cooling reverse donor-to-acceptor transition takes place

Effect of thermal annealing on the luminescent characteristics of CdSe/ZnSe quantum dot heterostructure

Effect of post-growth thermal annealing within the temperature range 200 to 430 ºC for 15 min on the luminescent characteristics of CdSe/ZnSe quantum dot (QD) heterostructure was studied. Annealing at lower temperatures (Tann <= 270 ºС) results in an increase by a factor of 2-3 of the intensity of two photoluminescence bands observed, the first being caused by excitonic transitions in QDs and the second one being connected with the defect complex including a column II vacancy. The effect is supposed to be caused by annealing of as-grown nonradiative defects. Annealing at higher temperatures (Tann > 270 ºС) stimulates a decrease of the QD photoluminescence band intensity and up to 100 meV blue shift of its peak position. The former is explained by generation of extended defects and reduction of the QD density. The blue shift observed at 370-430 ºС is ascribed to diffusion of cadmium from QDs that also results in reduction of the QD density. It is found that the energy of excitonic transitions in the wetting layer does not change upon annealing. Lower thermal stability of QDs as compared to that of the wetting layer has been explained by strain-enhanced lateral Cd/Zn interdiffusion via vacancies. The presence of column II vacancies in the wetting layer is proved by characteristics of defect-related PL band and its excitation spectra

Defects and radiation-enhanced defect reactions in ZnSe/(001)GaAs MBE layers

Optical and structural properties of undoped ZnSe epilayers with thickness ranging from 0.5 to 2 mm grown by molecular beam epitaxy on GaAs (001) substrates have been investigated by depth resolved optical and X-ray methods. It was found that the epilayers with thicknesses above some value (>1 μm) contain three regions of different structural and optical quality. It is shown that two of these regions (near top surface and near interface ones) contain higher defect density. The nature of luminescence line at 446.1nm (4.2 K) is discussed. It was found that the radiation enhanced defect reactions occurred in the top surface region of epilayer

Influence of cation vacancy related defects on the self-assembly processes in CdSe/ZnSe quantum dot heterostructures

A spatial distribution of the cation vacancy related defects and their influence on the formation processes of self-assembled nanoislands in CdSe/ZnSe heterostructures were investigated by photoluminescence methods. Self-assembling growth was achieved under low temperature (2300C) molecular beam epitaxy with a subsequent annealing step. To change a number of cation vacancy related defects a VI/II beam pressure ratio РVI/РII was varied from 2:1 to 5:1. In the samples grown under РVI/РII = 5:1 a significant increase of self activated emission band caused by cation vacancy related defects was found. A study of the excitation spectra of defect related band revealed that in all samples the cation vacancy related defects are present in ZnCdSe wetting layer. In the samples grown under РVI/РII = 5:1 they were observed on nanoisland interface too. It was found that the increase of Se beam pressure results also in high energy shift and narrowing of nanoisland emission band. This process is accompanied by low energy shift of ZnSe band-to-band emission. Observed changes in photoluminescence spectra are explained by the decrease of Cd content in ZnCdSe layer due to enhancement of Cd/Zn interdiffusion process in the result of the increase of vacancy related defect number

Deep-level defects in CdSe/ZnSe QDs and giant anti-Stokes photoluminescence

CdSe/ZnSe structures with a quantum dot extrinsic photoluminescence band related to the defects that contain vacancies in cation sublattice has been investigated. It is shown that such defects can be localized in different parts of heterostructure (inside ZnSe barrier and cap layers, Zn₁-xCdxSe wetting layer and at quantum dot heterointerface) and their localization depends on the preparation regimes and parameters of investigated structures. It is shown that defect level follows the heavy-hole related level. An intense anti-Stokes photoluminescence of quantum dots has been found. Two-step excitation mechanism of the anti-Stokes photoluminescence through the local states of investigated defects localized on the quantum dot interface is proposed

Influence of annealing on luminescence and energy transfer in ZnO multilayer structure co-doped with Tb and Eu

International audienceThe influence of rapid thermal annealing (RTA) on structural and optical properties of ZnO multilayer structures co-doped with Tb and Eu ions has been investigated by X-ray diffraction, Time-of-Fligth Secondary Ion Mass Spectrometry, Specular Infrared reflection, micro-Raman and photoluminescence (PL) methods. It is shown that incorporation of rare earth (RE) ions in ZnO host is accompanied by the formation of intrinsic defects in oxygen and zinc sub-lattices of ZnO. The appearance of intense Raman mode at 275 cm−1 is ascribed mainly to Eu ion incorporation on Zn site in ZnO matrix. The PL of RE ions localized in ZnO and other crystal phases is revealed. The effect of energy transfer from Tb3+ to Eu3+ ions in ZnO is identified. It is shown that the RTA improves crystal structure of ZnO host, i.e. stimulates the increase of coherent domain sizes and strain relaxation, as well as promotes the redistribution of RE ions across the structure. The effect of RTA on RE ion PL depends strongly on the annealing temperature. It is found that RTA at 500 °C promotes Eu incorporation into ZnO and the enhancement of Eu3+ PL due to energy transfer from Tb3+ to Eu3+, while RTA at 800 °C stimulates segregation of RE ions and the decrease of their PL