Cathodoluminescence of zinc and ytterbium oxide poly- and single crystals grown from melt under high pressure

A mixture of poly- and single crystals of zinc and ytterbium (2 at.%) oxides have been grown from the melt at high temperature (1430 o C) and high pressure (3.8 GPa). The crystals were transparent under examination with naked eye. The diameter of crystals was in the range from 0.005 to 2 mm. X-ray diffraction confirmed presence of individual zinc and ytterbium oxides in recovered samples. No change of zinc oxide lattice parameters was revealed comparing to pristine zinc oxide. Cathodoluminescence spectra of the mixture were recorded at 77 and 293 K. The collected spectra exhibit UV, green and near-infrared bands due to exciton recombination, presence of oxygen vacancies and ytterbium ions in ZnO crystals, respectively.


INTRODUCTION
Zinc oxide is one of the most perspective semiconductors for solar cell up-and downconverters because it's cheap and abundant host for lanthanides [1,2]. The hosted lanthanides can effectively convert UV and visible light into near-infrared range where the c-Si solar cells strongly absorb light [3][4][5][6]. The light conversion can occur through non-radiative energy transfer from excited state of ZnO to the excited states of lanthanide ions with subsequent light emission in the infrared range [7][8][9]. The energy transfer is quite effective process resulting in the efficiency up to 200% when quantum cutting takes place [10].
A number of lanthanides doped ZnO materials, such as composites, films, nanoparticles etc., has already reported in literature [7][8][9][11][12][13]. However, it's still unclear whether large number of lantanide ions can be dispersed in ZnO crystalline lattice or the ions can only segregate into crystalline lanthanide oxide due to large charge and radius of trivalent lanthanide ion. Such thermodynamically non-equilibrium method as ion-beam implantation results in incorporation of lanthanide ions in ZnO crystalline lattice with its subsequent out diffusion and segregation upon heat treatment [14,15]. That fact also raises a question whether the luminescent lanthanide centers located in the ZnO or at interface between zinc and lanthanide oxides [7][8][9].
This work is a follow-up of our previous article on ZnO single crystal growth via spontaneous crystallization from high-temperature melts under high pressure [16]. In that work, the grown ZnO single crystals (up to several millimeters) exhibited strong luminescence in the UV and visible ranges. In the present work, the crystal growth method was adapted for growing a mixture of poly-and single crystals of ZnO and Yb 2 O 3 , and the recovered samples were studied by scanning electron microscopy (SEM), X-ray diffraction and cathodoluminescence spectroscopy.

EXPERIMENTAL
A mixture of poly-and single crystals of zinc and ytterbium (2 at.%) oxides have been grown from the melt at 3.8 GPa in a toroid-type high-pressure apparatus [16]. The details of the growth method are described elsewhere [17,18]. The morphology and elemental composition of the samples were examined using a LEO Supra 50 VP (Carl Zeiss) high resolution scanning electron microscope equipped with energy dispersive X-ray detector.
The single crystal and powder X-ray diffraction studies were performed using STADIVARI PILATUS100K equipped with a semiconductor detector. The collected patterns were analyzed with WINPLOTR software.
The cathodoluminescence (CL) spectra were measured with a custom-made setup equipped with a pulsed "electron gun" and vacuum cryostat (510 -7 mm Hg). The spectra were recorded using a diffraction grating spectrometer (dispersion 5 Å/mm) and synchronous detector at temperatures of 77 and 300 K. The current and energy were varied within 0.05-2 mA and 10-50 keV ranges, respectively.

RESULTS AND DISCUSSION
A difference of heat dissipation by a top plate and a low piston of hydraulic press results in the   1(a,b) show SEM images of single and multiple ZnO:Yb 2 O 3 crystals. One can see large irregular crystals of ZnO with diameters varying from tens of micrometers to 0.1 mm. The small inclusions of Yb 2 O 3 crystals are seen on the surface of large ZnO crystals (Fig. 1c). The SEM image of initial Yb 2 O 3 crystals is shown in Fig. 1d. They are irregular with diameters much smaller than 1 μm. After crystallization from the melt, Yb 2 O 3 crystals become enlarged and grow   ZnO crystals were determined from powder and single-crystal X-ray diffraction data ( Table 1).
The obtained values are in good agreement with ICDD PDF-2 [36-1451] card for pure ZnO indicating a low doping level of ZnO by Yb.  Fig. 4 shows cathodoluminescence spectrum of undoped ZnO single crystal at room temperature.
The excitation of zinc oxide with electrons results in typical UV and green emission bands located in the ranges 350-430 nm and 450-600 nm, respectively. The UV emission is due to radiative recombination of free and bound excitons [19]. The green emission band is associated with various intrinsic defect centers, such as oxygen or zinc vacancies, in ZnO emerging during the synthesis [20].

CONCLUSIONS
The method of crystal growth from the melt at high pressure was applied for growth of a mixture of poly-and single crystals of zinc and ytterbium oxides. SEM examination revealed that small