Spectroellipsometric and photoluminescent studies of SnO_x nanostructures doped with Sm ions

Abstract

We present results on the photoluminescence and optical properties of SnO_x nanocrystals as a function of their size. Samariums ions are incorporated in SnO_x nanostructures to huge improve their photoluminescence efficiency. We have performed photoluminescent, ellipsometrical, atomic force microscopy, and structural studies of these nanocrystals and found them to be a direct semiconductor with a gap of ≈3.6 eV. The origin of the observed photoluminescence has been discussed. The doping of lanthanide atoms in semiconductor nanocrystals expands the range of possibilities offered by such materials, allowing them to be modified to meet specific requirements in electronic and optoelectronic applications.

Introduction

During the past two decades, the photoluminescence (PL) of semiconductor nanocrystals has been discussed intensively [1], [2], [3]. Investigation of the optical properties of semiconductor nanoparticles is important for both fundamental research and applications. Today, for example, semiconductors based on SnO₂ are the important materials for the microelectronics industry. SnO₂ is transparent semiconducting material with a tetragonal rutile structure and a wide band gap of around 3.6 eV and it is mainly used as gas sensors, transparent conductor, in photovoltaic cells, photocatalysis and optical materials [4]. Due to its large band gap and exciton binding energy, it is attractive as light emitters for the ultraviolet-visible spectral range. Nanostructures formed from wide band gap semiconductors are especially interesting and have potential use as luminescent materials in solid-state lighting [1], [3]. For such applications, the oxygen vacancy related defect band can be used for down-conversion of excited UV or blue light into red region of the spectrum. Due to the reduction of the nonradiative contribution to the relaxation mechanism and the high photoluminescence efficiency because of a low cutoff vibrational energy (∼600 cm⁻¹) the SnO₂ as a wide band gap semiconductor has huge attraction. Generally lanthanide ions such as Eu³⁺, Er³⁺ and Sm³⁺, etc., are incorporated in SnO₂ to improve the photoluminescence (PL) efficiency [3], [4], [5]. The luminescence in such structures is mainly due to the transitions of electrons between the spin-orbital splitted 4f states of the rare-earth atoms entering optical active centers. The ions of rare-earth elements incorporated in SnO₂ nanocrystals can also be selectively excited by energy transfer from the host matrix to the Eu³⁺, Er³⁺ and Sm³⁺ ions. It was shown that the Eu³⁺ (or Sm³⁺) emission can be strongly enhanced by energy transfer from the SnO₂ nanocrystals [3], [5].

While SnO₂ pure nanostructures have attracted a lot of attention, the potential of such nanostructures activated by rare-earth Sm³⁺ ions has not been fully exploited yet. It is of great interest to distinguish between the luminescence arising from Sm³⁺ ions and of that arising from defect centers in the SnO_x matrix. Thus, the purpose of this paper is to study in detail the structural and the complex optical properties of SnO_x and SnO_x:Sm nanoparticles. The complex refractive index and absorption coefficient of the SnO_x-based nanostructures in the photon energy range of 1–5 eV have been determined with spectroscopic ellipsometry (SE). The results are discussed considering the effect of the nanocrystal size on the band gap altering. The dependences of the emissions originated from defects in SnO_x and Sm³⁺ ions in SnO_x:Sm on the SnO₂ nanocrystals size are found. The relation between the PL dependences and the absorption features has been confirmed. Our results demonstrated the strong influence on the PL spectra the absorption edge and additional states inside the band gap of SnO₂ semiconductor. The optical measurements have also provided evidence for a defect-related trap level that may be involved in the energy transfer between the SnO_x host and the Sm³⁺ ions.