Elsevier

Acta Materialia

Volume 196, 1 September 2020, Pages 626-634
Acta Materialia

Full length article
Understanding the UV luminescence of zinc germanate: The role of native defects

https://doi.org/10.1016/j.actamat.2020.07.009Get rights and content

Abstract

Achieving efficient and stable ultraviolet emission is a challenging goal in optoelectronic devices. Herein, we investigate the UV luminescence of zinc germanate Zn2GeO4 microwires by means of photoluminescence measurements as a function of temperature and excitation conditions. The emitted UV light is composed of two bands (a broad one and a narrow one) associated with the native defects structure. In addition, with the aid of density functional theory (DFT) calculations, the energy positions of the electronic levels related to native defects in Zn2GeO4 have been calculated. In particular, our results support that zinc interstitials are the responsible for the narrow UV band, which is, in turn, split into two components with different temperature dependence behaviour. The origin of the two components is explained on the basis of the particular location of Zni in the lattice and agrees with DFT calculations. Furthermore, a kinetic luminescence model is proposed to ascertain the temperature evolution of this UV emission. These results pave the way to exploit defect engineering in achieving functional optoelectronic devices to operate in the UV region.

Introduction

Wide bandgap semiconductor oxides, due to their structural and electronic properties, have emerged as key materials for efficient ultraviolet (UV) absorption and/or emission while being transparent to the visible light. Light-matter interaction in the UV range is a challenge in a broad range of applications, such as solar-blind photodetectors [1], [2], solid-state lighting [3], sensors for health or environmental monitoring [4], to name a few. For these purposes, ZnO (Eg=3.4eV), SnO2 (Eg=3.1eV) and TiO2 (Eg=3.5eV) have been intensively investigated in the last decades, but their bandgap energy Eg limits somehow going deeper in the UV region. Novel ultra-wide bandgap materials, such as Ga2O3 [5], [6] or alternatively ternary oxides, can overcome to some extend this drawback, enabling even bandgap engineering with wider bandgaps [7]. In this sense, zinc germanate Zn2GeO4 with Eg ∼ 4.5eV at room temperature, has recently been envisaged as a promising semiconducting oxide due to its intrinsic physical properties in terms of good electronic conductivity, optical transparency or chemical stability [8]. The interest in this ternary oxide has already fostered new ideas in the design of efficient phosphors [9], high-performance solar blind photodetectors [10] and batteries [11] that incorporate Zn2GeO4 nanoparticles and/or nanowires as active elements. However, there is still a lack of understanding of the UV light absorption and emission processes in connection to their microstructure, including their native defects structure, which is crucial to fully exploit its potential in UV optoelectronic devices.

Luminescence from semiconducting oxides usually present broad visible emission bands often controlled by donor-acceptor pair (DAP) radiative recombinations, in which oxygen vacancies play the major role as donor levels while cation vacancies are responsible for the acceptor levels [12], [13], [14]. Visible emission bands have also been reported in Zn2GeO4 and attributed to the above mentioned DAP recombination processes in a general way [15], [16]. Moreover, its luminescence has been found to be influenced by the synthesis route of the material, signaling that not only vacancies but also interstitial defects could play an active role in the optical response [15], [17]. Also, the observation of both visible photoluminescence (PL) and UV cathodoluminescence (CL) bands from high quality microrods of Zn2GeO4 obtained by a thermal evaporation method [18] suggests that either UV or visible emission is feasible in Zn2GeO4 under certain excitation conditions, i.e. via electrons or photons. The above findings have not been yet found a satisfactory explanation, and therefore, an in-depth comprehension of the electronic states induced by specific native defects is required.

Indeed, one of the appealing features of wide bandgap semiconductor oxides is their potential to interact with UV light. However, UV emission is rather difficult to achieve in undoped oxides because native defects induce traps that quench near band edge transitions. One way of luminescence tailoring is by doping with optically active impurities, which provides emission at specific wavelengths depending on the impurity of choice. To this end, rare-earth ions are very suitable due to their well-defined intraionic levels transitions. However, doping with heavy ions is not always straightforward because of the low ions diffusivity in the host crystal, and ion implantation methods might be needed, as reported in Gd3+ implanted Ga2O3 to achieve UV emission lines peaked at 313nm [19]. Besides, other issue for effectively doping oxides is the location of the impurity in the crystalline lattice and its interaction with native point defects, which could alter the electronic states in the material. In complex oxides, such as ternary ones, multiple options of intrinsic point defects happen with the concomitant creation of electronic traps in the bandgap. Hence, they would provide a valuable way of tuning the luminescence emission with no need of importing foreign cations. In particular, the crystalline structure of Zn2GeO4, built up by corner-shared ZnO4 and GeO4 tetrahedra arranged in rings of size enough to accommodate self-interstitial cations, would provide extra recombination pathways for excess carriers and be eventually responsible for the observed luminescence bands. To the best of our knowledge, a careful study of the PL emission as a function of photon energy excitation and the temperature evolution has not been fully addressed in Zn2GeO4. Moreover, although some works have reported first-principle calculations of electronic states in Zn2GeO4 crystals [20], [21], there is still a lack of a thorough description of the electronic levels related to native vacancies and Zn interstitials in this ternary oxide. Therefore, in order to get an accurate picture of the optical properties and to ascertain the origin of the UV emission bands of Zn2GeO4, experimental and theoretical work is needed to correlate the nature of point defects with the observed luminescence properties.

In this work, we provide a comprehensive study of the UV emission of Zn2GeO4 by considering experimental PL results combined with first-principle calculations and theoretical modeling of Zn2GeO4 micro- and nanowires. We have found that UV emission exhibits a composed nature, with several components that can be separately excited by selective excitation. The experimental results have been compared with density functional theory (DFT) calculations, which have led us to suggest a theoretical model based on rate equations for the origin of these emissions in connection to electronic levels related to vacancies VO, VGe, VZn and interstitial Zni native defects. The results of this work would also be of major interest for applications since they pave the way to exploit defect engineering in achieving functional optoelectronic devices operating in the near and medium UV region.

Section snippets

PL and PLE measurements

Herein, Zn2GeO4 micro-wires have been synthesized by a catalyst-free thermal evaporation method as described in the methodology section. The microstructural analysis of the microwires has been assessed by Raman spectroscopy and X-ray diffraction measurements and it has been already reported [18], [22]. This procedure allows us to grow material of high crystalline quality. Fig. 1(a) shows a representative SEM image of the network of Zn2GeO4 structures under study. Fig. 1(b) shows the room

Conclusions

The UV luminescence bands of Zn2GeO4 have been in-depth studied as a function of temperature and excitation energy conditions. The relationship between native defects and electronic states has been explored by means of DFT to ascertain the origin of the observed luminescence bands. A broad and a narrow UV emissions have been recorded and associated to oxygen vacancies-related levels and zinc interstitials-related levels, respectively. The particular features of the N-UV emission have motivated

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported by Ministerio de Ciencia, Innovación y Universidades (Grants MAT2016-75955 and RTI2018-097195-B-I00) and Deutsche Forschungsgemeinschaft (Grant CU 44/47-1). R. M.-C. would like to thank Dr. Giuseppe Mallia for very useful discussions.

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