Optical Signatures of Impurity–Impurity Interactions in Copper Containing II–VI Alloy Semiconductors

Abstract

We study the optical properties of copper containing II-VI alloy quantum dots (CuZnCdSe). Copper mole fractions within the host are varied from 0.001 to 0.35. No impurity phases are observed over this composition range, and the formation of secondary phases of copper selenide are observed only at x > 0.45. The optical absorption and emission spectra of these materials are observed to be a strong function of x, and provide information regarding composition induced impurity-impurity interactions. In particular, the integrated cross section of optical absorption per copper atom changes sharply (from 1 × 10 nm to 4 × 10 nm) at x = 0.12, suggesting a composition induced change in local electronic structure. These materials may serve as model systems to understand the electronic structure of I-III-VI semiconductor compounds.

Optical Properties and Electronic Structure of Copper Zinc Sulfide Nanocrystals

“…In addition, the limited spatial extent of the Cu impurity levels within the host leads to a pronounced high HMA character for Cu−II−VI alloys studied thus far. 24 This also ensures the strong similarity of the emergent optical properties of Cu-related transitions across hosts.…”

“…These effects are similar to the observations made in the case of CuCdZnSe materials where Cu has been demonstrated to be prone to interimpurity interactions that lead to considerably broadened energy level distribution of the impurity. 24 This further leads to the emergence of a strong absorption band in the visible−NIR region. In addition, the limited spatial extent of the Cu impurity levels within the host leads to a pronounced high HMA character for Cu−II−VI alloys studied thus far.…”

“…In addition, Cu has also been shown to alloy into II−VI hosts leading to the emergence of an unusual class of highly mismatched alloys. 24 Previous reports have described the alloying of Cu into CdZnSe and ZnSe hosts. 25,26 In the latter case, Cu has been shown to introduce d levels into the host bandgap, giving rise to an absorption tail as well as composition tuned spontaneous emission rates.…”

“…ZnS. As with selenide hosts, we have already reported 24 that high inclusions of Cu, i.e., with x Cu as high as 0.45 (defined in a cation basis), may be achieved without the formation of independent selenide phases. However, here we report the maximum incorporation of copper as x Cu = 0.18 in the Cu x Zn (1−x) S (CZS) nanocrystals (NCs) which exhibit a strong near-infrared absorption with bandgaps as low as 2.15 eV.…”

Frontier challenges in doping quantum dots: synthesis and characterization

“…However, it was soon realized that it is not possible to seamlessly include dopant ions within the host lattice, although researchers have recently demonstrated the inclusion of high percentages of dopants with appropriate controls using stoichiometric addition. 34 While doping these QDs, only a small fraction of the added dopants is incorporated into the crystal lattice, while a large proportion of the dopants remains on the surface or forms clusters. 35,36 Energetically, it is more favorable to form clusters of dopant ions, segregate into a separate phase or simply be adsorbed on the host surface.…”