Cathodoluminescence of zinc oxide crystals grown from melt under high pressure in the presence of ytterbium oxide

A mixture of poly- and single crystals of zinc and ytterbium (2 at.%) oxides have been grown from the melt at high temperature (1430  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 the recovered samples. No change of zinc oxide lattice parameters was observed compared 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 promising semiconductors for solar cell up-and downconverters because it's cheap and appropriate host for lanthanides [1,2]. The lanthanides atoms on ZnO surface 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 an effective process resulting in the efficiency of up to 200% when quantum cutting takes place [10].
A number of lanthanide-doped ZnO materials, such as composites, films, nanoparticles etc., has already been 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 the 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 ZnO matrix 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.

MATERIALS AND METHODS
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]. Microcrystalline powders of ZnO (Alfa Aesar, 99,9995%) and Yb 2 O 3 (Alfa Aesar, 99,99%) were thoroughly mixed and ground in a mortar, then pressed into a pellet and placed into a special gold capsule. The ZnO:Yb 2 O 3 molar ratio was equal to 50:1. The samples were gradually compressed up to 3.8 GPa and heated up to 1430C with an average rate 500C/min, annealed at this temperature for 2 min and cooled down to 1100C with an average rate 90C/min. Crystallization of the melt was observed in the temperature range from 1290 to 1250C, as evidenced by an increase of electrical resistance of the high-pressure cell. Finally, the sample was quenched down to room temperature and slowly decompressed. The recovered pellet comprised of a mixture of transparent greenish-yellow crystals with diameters from 0.005 to 0.2 mm.
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 an energydispersive 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 varied within 0.05-2 mA and 10-50 keV ranges, respectively.

RESULTS AND DISCUSSION
The peculiarities of heat dissipation by a top plate and a low piston of in the hydraulic press result in the temperature gradient in high-pressure cell of the toroid-type apparatus. The value of the temperature gradient is approximately 10-20C/mm at 1430C and 3.8 GPa. That temperature gradient plays a significant role in the process of ZnO crystal growth from the melt.
Upon quenching, the crystallization process immediately starts in the top relatively cold part of the cell resulting in a disordered layer of ZnO polycrystals. Further, these polycrystals act as seeds for the crystal growth. Upon further temperature decrease, the ZnO crystals continue to grow along the temperature gradient forming a batch of needle-like crystals.
The crystal growth process changes in the presence of Yb 2 O 3 in the initial mixture. Under high pressure and temperature, Yb 2 O 3 dissolves in the ZnO melt. Upon quenching Yb 2 O 3 crystallizes first forming a layer of polycrystals. The Yb 2 O 3 polycrystals act as seeds for subsequent growth of ZnO single crystals. Fig. 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 on the surface of ZnO single crystals. Therefore, the obtained SEM images confirm our hypotheses on the change of crystal growth process after addition of Yb 2 O 3 crystals to the melt.  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]. luminescence decreases while the intensity of green luminescence is virtually unchanged. The infrared band emerges due to radiative electron transition from 2 F 5/2 excited state to 2 F 7/2 ground state in Yb 3+ ions [7][8][9]. The intensity of the infrared cathodoluminescence drastically increases upon cooling down the single crystals down to liquid nitrogen temperature. The infrared luminescence band slightly red-shifts, possibly due to the change in electron population of the excited and ground states.

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