Growth of Nd:Cr:YVO4 Single Crystals by the Optical Floating Zone

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Growth of Nd:Cr:YVO4 Single Crystals by OFZ Technique under Different Oxygen Partial Pressure to Control the Oxidation State of Chromium Indranil Bhaumik, S. Ganesamoorthy, Rajeev Bhatt, Amit Saxena, Ashwani Kumar Karnal, and P. K. Gupta Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg400506d • Publication Date (Web): 18 Jul 2013 Downloaded from http://pubs.acs.org on July 19, 2013

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Crystal Growth & Design

Growth of Nd:Cr:YVO4 Single Crystals by OFZ Technique under Different Oxygen Partial Pressure to Control the Oxidation State of Chromium Indranil Bhaumik, Sarveswaran Ganesamoorthy†, Rajeev Bhatt, Amit Saxena, Ashwani Kumar Karnal* and Pradeep Kumar Gupta Laser Materials Development and Devices Division, Raja Ramanna Centre for Advanced Technology, Indore 452 013, India. †

Materials Science Group, IGCAR, Kalpakkam 603102, India.

A very recent report by Pan et al. [Optics Express, 2012] has introduced Nd:Cr:YVO4 as a suitable substitute of Nd:Cr:YAG, which has certain disadvantages, as self Q-switched laser medium. However, the crystals were grown by Czochralski method and had significant amount of undesired Cr in +3 state. Reduction of Cr5+ to Cr3+ is due the oxygen vacancy present in the lattice. Our present investigation provides a recipe to increase the desired +5 oxidation state of Cr as well as decrease the undesired Cr3+ by increasing the oxygen content in the growth ambience which is possible to control in techniques like optical floating zone method. Further a red shift in the band-gap energy observed which is understood to be due to the incorporation Nd in the lattice whereas Cr does not influence the band-gap energy.

(b)

(a) (c)

(a) Image of melt volume during growth experiments showing the instability in the melt. (b) Asgrown crystal. (c) Dependence of absorption on the oxygen content in the growth ambience.

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Growth of Nd:Cr:YVO4 Single Crystals by OFZ Technique under Different Oxygen Partial Pressure to Control the Oxidation State of Chromium Indranil Bhaumik, Sarveswaran Ganesamoorthy†, Rajeev Bhatt, Amit Saxena, Ashwani Kumar Karnal* and Pradeep Kumar Gupta Laser Materials Development and Devices Division, Raja Ramanna Centre for Advanced Technology, Indore 452 013, India. †

Materials Science Group, IGCAR, Kalpakkam 603102, India.

Corresponding Author *Fax: +91-731-2488650, e.mail: [email protected]. ABBREVIATIONS OFZ, Optical Floating Zone; YVO4, Yttrium ortho-vanadate; Cz method, Czochralski method.


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Abstract

Here we address the issue of growing single crystal of Nd and Cr co-doped YVO4, a promising self Q–switching laser material, in different oxygen partial pressure. Single crystals of 1.4% Cr and 0.7% Nd co-doped were grown by optical floating zone (OFZ) technique. Our findings emphasize, as the oxygen content in the growth atmosphere is increased, the absorption band corresponding to 2A1→ 2B2 transition of Cr5+, which participates in self-Q-switching, improves significantly. Similarly a significant improvement is observed in the present case in comparison to that grown by Czochralski method in 2% oxygen by Pan et al., [Optics Express, 2012]. Crystals grown by OFZ technique contain lesser oxygen vacancies that inhibit the tendency of Cr ions to get reduced to the undesired +3 state. Red shift in the band-gap energy is observed due to the incorporation of Nd whereas Cr does not influence the band-gap energy.

Keywords Laser host material, Yttrium ortho-vandate, Crystal growth, OFZ method, Optical absorption.


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INTRODUCTION Yttrium ortho-vanadate (YVO4) doped with rare earth Nd is widely used for compact diode pumped solid state lasers as it offers several advantages like high slope efficiency, low laser threshold, wide pumping bandwidth and linearly polarized emission.1-4 In recent years, a lot of research has focused on the development of laser crystals with multi-functionality, such as the self-frequency-doubling, self-mode locking, self Q-switching etc., as these can lead to compactness, low loss, and simplicity in the laser design. In the context of self-Q-switching, neodymium (Nd3+) and chromium (Cr4+) co-doped YAG have been identified to be an efficient material.5 However, as Cr4+ ion substitutes a fraction of Al3+ ions in YAG lattice, for charge balance co-doping with divalent ions like Ca2+ or Mg2+ ions is necessary.6 This leads to complexities both in terms of growth of homogeneous crystal as well as its application.7 To resolve this problem, very recently Pan et al.7 explored Cr co-doped Nd:YVO4 crystals grown by Czochralski (Cz) method (earlier also grown by Matrosov et al. by Cz method8) as an alternative of Nd:Cr:YAG for self-Q-switched laser. In the case of YVO4 Cr5+ ions substitute the tetrahedrally coordinated penta-valent vanadium ions. Therefore, unlike YAG, Cr substitution in Nd:YVO4 does not result in any charge imbalance in the medium. Subsequently Pan et al. have also demonstrated the self Q-switched laser performance in this crystal. A limitation of Cz method used by Pan et al. for the growth of Nd:Cr:YVO4 in iridium crucible is that the grown crystals have high level of oxygen deficiency as more than 2% oxygen cannot be used in the growth ambience. This leads to a reduction of Cr5+, which is required for self Q-switching application, to undesired low oxidation state Cr3+. Consequently the efficacy of the material is greatly hampered. Here we report the growth of Nd:Cr:YVO4 in oxygen rich ambience using optical floating zone technique because it does not have limit on the use of oxygen as it is a


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crucible-less technique. The effect of growth ambience on its absorption characteristics especially in the context of pentavalent Cr state is further investigated. The effect of annealing in oxygen atmosphere on the chromium absorption band is also discussed. EXPERIMENTAL DETAILS The crystals were grown by optical floating zone (OFZ) method at atmospheric pressure with flow rate of 1 liter per minute for all the growth experiments with different oxygen content. Doping concentration for Nd and Cr were 0.7 at% and 1.4 at% respectively. For the growth, starting chemical was synthesized from Y2O3, V2O5, Cr2O3 and Nd2O3 with 99.99 % purity. Proper phase formation is a very important step especially for the growth of crystal having volatile components.9 Synthesis of YVO4 has a tendency to form secondary phase because of volatility of V2O5. Our earlier studies show that addition of 3 mol% V2O5 powder in excess for compensating the V2O5 loss leads to single phase formation.10 Initial starting chemicals (Y2O3=10.990g, V2O5=9.059g, Cr2O3=0.104g and Nd2O3=0.116g) for 20 g of total charge were mixed in a turbo three dimensional mixer for 3 h. The mixed powder was heated at 650 °C for 12 h for solid state reaction to yield YVO4 from the initial precursor. Phase formation was confirmed from XRD pattern of the synthesized powder. Feed-rods needed for the growth were prepared from the synthesized charge using cold hydrostatic press at 70 bar pressure, which were then sintered at 1200 °C for 10 h for densification. To obtain uniform sintering density the rod was provided rotation and continuous up-down translation inside the hot zone of the sintering furnace. Growth of single crystal was carried out in an optical image furnace (Crystal Systems Corp., Japan) equipped with four 1 kW halogen lamps focused by four ellipsoidal mirrors. Four lamp based system offers significantly better thermal homogeneity compared to two mirror based system.11,12 For the present growth


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experiments [100]-oriented seeds were used. For YVO4 the lasing property is superior along this direction.1 As the maximum diameter of good quality crystal that can be grown by this method is 5-6 mm, for fabricating laser element of appreciable length (5-10 mm) one needs to grow the crystal along the same direction. The grown crystals along this orientation had elliptical crosssection as discussed in ref. 10. The growth conditions were optimized by performing a number of growth experiments. Growth rate of ~8 mm/h resulted in the best optical quality crystals. For investigating the effect of oxygen content in the growth ambience crystals were grown in air as well as a mixture oxygen and nitrogen of ratio 50:50 and 75:25, and full oxygen. To check the homogeneity of the grown crystal a [001] oriented sample was cut and polished, and conoscopy pattern was observed under diverging light in cross-polar condition using Olympus BX-60 polarized light microscope. To investigate the effect of Cr incorporation the lattice powder XRD pattern of the grown crystal was recorded. The XRD data was recorded using Cu-Kα in the range 20-70o with step of 0.02° by rotating anode (Ultrax-18) diffractometer in Bragg-Brentano geometry working at 40 kV and 50 mA. Rocking curve analysis was carried out using Panalytical X’part PRO in the 4 bounce geometry. To estimate the dopant concentrations in the grown crystals EDAX measurement was carried out in Zeiss Supra-55 SEM with EDAX attachment. To study the optical properties polished plates of thickness 1 mm were used. Absorption measurements were carried out at room temperature using JASCO V670 spectrophotometer in the range of 300 to 1500 nm with step of 0.1 nm.


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RESULTS AND DISCUSSION Growth of crystal and melt stability The molten zone length is one of the important parameters in the crucible-less zone melting growth. The stability of the molten zone is governed by surface tension of the liquid and the rotation rates of the feed rod and the seed rod. Under un-optimized conditions YVO4 molten zone suffers from severe overflow problem. This makes it difficult to maintain a fixed diameter of the crystal at a constant heating power and fixed growth rate. Normally, the solid liquid interface is kept concave for the stability of the molten zone and its shape depends on the rotation rate (with no option to change the thermal gradient in the growth equipment). For the growth along [100] as the cross-section is elliptical during the growth of the crystal the horizontal cross-section of the melt is elliptical. This further complicates the stability issue of melt as the feed rod and growing crystal are rotating in the counter direction. Fig. 1 shows a view of the melt along the (i) major axis and (ii) minor axis of elliptical cross-section of the crystal during growth. Significant difference in the extent of the melt along these two horizontal directions can be seen that leads to instability. The input power and the growth/feed rate have to be carefully controlled to avoid such instability (Fig. 1, iii).


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Figure 1. Melt volume during growth experiments (from left to right): (i) view along major axis of the elliptical cross-section of the growing crystal and (ii) view along minor axis of the elliptical cross-section of the growing crystal at the same instance. (iii) A view along minor axis showing unstable melt volume. For the growth, rotation rate was optimized to be 30 rpm in counter directions for the seed and the feed-rod. It was observed that the convexity of the interface decreased with increasing rotation rate. After growth, the crystal was slowly cooled to the room temperature. Crystals of diameter ~5-6 mm and length 20-30 mm were grown. Fig. 2(a) shows as-grown crystals. The observed circular concentric fringes in the conoscopy pattern [Fig. 2(b)] confirm the homogeneity of the sample. To check the concentration of the doping EDAX was carried out on the grown samples. The concentration of Cr for the grown crystals was found to be 1.27-1.34 at% on the average obtained at three different locations on each sample. The concentration of Nd is found to be 0.62-0.70 at%.


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(b)

Figure 2. (a) As-grown 1.4 at.% Cr and 0.7 at% Nd co-doped YVO4 crystal under air flow (top) and 50% oxygen and 50% nitrogen flow (bottom), (b) Conoscopy pattern of c[001]-cut sample. X-ray Diffraction and rocking curve analysis Fig. 3 shows the powder diffraction of pure, Nd doped and Nd-Cr co-doped YVO4 single crystals. Peaks were indexed using JCPDS data.13 The crystal possesses tetragonal structure with lattice parameters a= 7.11(3) Å and c= 6.29(5) Å. Comparing with the JCPDS data (a= 7.119 Å and c=6.292 Å) it can be concluded that the doping with Cr does not change the lattice parameters of YVO4. By ESR analysis14 it has been found that Cr ion replaces the V ion in tetrahedral coordination in YVO4 lattice. The ionic radii15 of Cr (48.5 pm in tetrahedral coordination) is close to vanadium (49.5 pm in tetrahedral coordination). On the other hand, Nd (112.3 pm in octahedral coordination) replaces the yttrium ion (104 pm in octahedral coordination). No change in the lattice parameters was observed for crystals grown under different oxygen ambience.


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To evaluate the crystalline quality of the grown crystals the rocking curve analysis was carried out on the a-cut samples. The sample was rocked (ω-scan) around the peak corresponding to (200) peak. The FWHM of the rocking curve is 34.6, 41.2, 38.7 and 32.5 arc-second for the crystal grown in air, 50%, 75% and 100% oxygen, respectively. The values are little bit higher than that of the Cz grown crystal as reported by Pan et al.7 but well in the acceptable limit for optical applications. This is expected for the crystal grown in optical floating zone technique which has high thermal gradient. Fig. 4 shows the rocking curve for crystal grown in 100% oxygen and air.

Figure 3. Powder XRD patterns of YVO4, Nd:YVO4 and Nd:Cr:YVO4


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Figure 4. Rocking curve around (200) peak for Nd:Cr:YVO4 crystal grown in 100% oxygen. Inset shows the rocking curve for the crystal grown in air. Absorption characteristics The absorption coefficient (α) was calculated from the transmittance spectra at room temperature, using the relation16: T ≈ (1 − R)2 exp(−α d )

where, T is transmittance, R is reflectivity (calculated from Sellmeier’s equation) and d is the sample thickness. Further a baseline correction is done to subtract the effect of scattering loss from the surface of the sample. Fig. 4 shows the absorption spectrum comprising several sharp absorption lines as well as broad bands of the crystal grown in air. The sharp transitions are due to the Nd ions. The initial state of all these transitions is the ground 4I9/2 state of the Nd3+ ions.17 The absorptions are due to a 4f-4f intra-band electric dipole (ED) transitions. The shielding by the outer shells of completely filled 5s2 and 5p6 is enough to make the f-f transitions sharp and distinct. On the other hand the broad absorption bands at around 400 nm, 650 nm and 1100 nm are due to Cr ions. The Cr5+ ions substitute for tetrahedral coordinated V5+ in the YVO4 which


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has a zircon structure. In the zircon structure, the tetrahedron is tetragonally elongated along the c axis so that the CrO43- site symmetry is D2d.18 The broad absorption band peaking at 400 nm is assigned to charge transfer transition.19 The absorption at 650 nm corresponding to the 2A1→ 2E transition is masked by the prominent absorptions of Nd ions in this wavelength range. Further there is a broad absorption band near the 1100 nm, which only appears when Cr ion in the lattice is in the penta-valent state.18 As shown in the energy level diagram of Cr5+ (3d1) ion [Fig. 5], this absorption band is due to the 2A1→ 2B2 transition. This absorption band enables the crystal to be a self-Q-switched laser matrix.7 The absorption coefficient around 1100 nm is 2.5 cm-1 and the full width at half maximum (FWHM) of the peak is about 218.8 nm.

Figure 5. Absorption spectrum of 1.4 at.% Cr and 0.7 at% Nd co-doped:YVO4. Absorption bands marked by asterix are of Cr ions. The inset shows the transition levels of Cr5+ ion.


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For comparison of Nd doped and Nd-Cr co-doped sample a normalized transmission plot of both the samples with same thickness is shown in Fig. 6. The absolute transmission percentage was almost same for both the samples. It is evident that the characteristic absorption lines of Nd ions are almost unchanged on Cr co-doping in terms of position and FWHM. The absorption coefficient corresponding to the absorption band at 1100 nm as claimed by Pan et al.7 is 2.27 cm-1. However, it is important to note that the plot as well as the calculation of absorption coefficient by Pan et al. contains a significant contribution of the scattering loss. After subtraction of the baseline in the absorption plot in reference 7, the absorption coefficient for 2

A1→ 2B2 transition for the crystal grown by Cz method7 is recalculated and found to be less than

0.5 cm-1 whereas the value of the absorption coefficient is 1.25 cm-1 (baseline subtracted) for the sample grown in air by OFZ technique. It is to be noted that the crystal grown by them is by Cz method where only 2% oxygen was used. To the best of our understanding the grown crystal in the presence of low oxygen would have oxygen vacancy and consequently, for charge compensation the Cr5+ in the tetrahedral site would have reduced to Cr3+ as per the equation give below. 1 OO → O2 + VO.. + 2e − 2

( generation of oxygen vacancy )

Cr 5+ + 2e − → Cr 3+

(reduction of Cr 5+ )

Here Oo refers to the oxygen in oxygen site and Vö refers to oxygen vacancy. On the other hand for crystal grown by OFZ in air atmosphere has lesser oxygen vacancy thus have relatively higher concentration of Cr5+ ions in the tetrahedral site and hence the absorption coefficient for absorption band around 1100 nm is higher.


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Figure 6. Normalized transmission spectra of 1.4 at.% Cr -0.7 at% Nd co-doped and 0.7 at% Nd doped YVO4. Effect of oxygen annealing on absorption spectra Further to confirm the effect of oxygen vacancy on the state of Cr ion, the plate cut from the crystal grown in air was annealed at 1000 °C under continuous oxygen flow for 24 h. Fig. 7 shows the absorption spectra of the annealed sample. Inset shows the absorption band near 1100 nm of both the pristine and the annealed samples and the Table 1 summarizes the quantitative change observed after annealing the sample in oxygen atmosphere. Peak height as well as the integrated area has increased on annealing. Also the FWHM was found to increase. So it is evident from the measurement that the absorption has increased, indicting the increase in the concentration of Cr ions in pentavalent state. As explained earlier it is possible that part of the Cr ion is at lower oxidation state inside the lattice because of oxygen vacancy. On annealing in oxygen atmosphere those can be converted to Cr5+ as per the equation below.


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1 O2 + VO.. + 2e − → OO ( removal of oxygen vacancy ) 2

Cr 3+ → Cr 5+ + 2e −

(oxidation of Cr 3+ )

Figure 7. Absorption spectrum of annealed sample. Inset: comparison between pristine and annealed samples for 2A1→ 2B2 transition.

Table 1. Parameters of 2A1→ 2B2 transition for crystals grown in air and annealed in oxygen.

Sample

Peak position (nm)

Integrated Area (x 10-7)

Height (cm -1)

FWHM (nm)

Pristine

1101.0

319.1

1.24

218.8

Annealed

1101.1

330

1.4

213


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Effect of oxygen ambience on the absorption spectra The above study confirmed that the oxygen vacancy plays crucial role in controlling the valance state of Cr ions. Thus it became imperative to grow and study crystals grown under different oxygen partial pressure. Fig. 8 shows the absorption spectra of a-oriented plates of thickness 1 mm of crystals grown in 50%, 75% and 100% oxygen along with that of the crystal grown in air. For comparison of the contribution of Cr+5 in the absorption, the 1100 nm absorption band is shown in inset after subtraction of the baseline to eliminate the other effect such as scattering loss. The peak was fitted using Gaussian function. The fit parameters obtained for all the crystal samples are tabulated in Table 2. The integrated area as well as the peak height found to increase with the increasing oxygen content in the ambience. The result as shown in Table 2 implies that the concentration of Cr ions in penta-valent state increases significantly. Referring to the model described in earlier section, the decrease in oxygen vacancy with the increase of oxygen content in the growth ambience prevents the conversion of Cr ions to be reduced to +3 states. The peak position and FWHM are almost unaltered. The result clearly emphasizes the fact that growing Nd:Cr:YVO4 in oxygen rich ambience helps in improving the absorption band centered around 1100 nm due to Cr5+ in the grown crystal.


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Figure 8. Absorption spectrum of the crystals grown in different oxygen atmosphere namely air, 25%, 75% and 100% oxygen. Inset: Comparison for 2A1→ 2B2 transition. Table 2. Peak parameters of 2A1→ 2B2 transition for crystals grown in different atmosphere Oxygen in percentage

Peak position (nm)

Integrated Area (x 10-7)

Height (cm -1)

FWHM (nm)

21 (air)

1101.0

319.1

1.24

218.8

50

1100.8

333.5

1.29

215.4

75

1101.3

351.0

1.37

218.1

100

1100.5

420.7

1.58

223.4


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As Cr band has absorption close to the band gap energy of YVO4, the effect of the Cr doping on the band gap of YVO4 is further investigated. YVO4 is a direct band gap material with the valence-band maxima and the conduction band minima are located at Γ point. Near the band edge,

the density of state can be approximated by parabolic band and the absorption spectrum is given by Tauc relation20, 21:

α hv ∝ (hv − Eg )1/2 where, hν is photon energy and Eg is the energy gap. The direct band gap energy was evaluated from the plot of α2 vs hν (Fig. 9a) for Nd:Cr:YVO4 crystal grown in 100% oxygen atmosphere. The abrupt rise of the absorption (α2) with incident photon energy near the band edge depicts the photon absorption due to direct allowed inter-band transition. The intercept of the straight line at the energy axis (α = 0) that is 3.6 eV yields direct optical band-gap (Eg) energy. The reported experimental value22 of band gap for YVO4 is ~3.8 eV which is higher than the value for Nd:Cr:YVO4 in the present case. However to ascertain whether it is an effect of Cr doping, band gap was estimated for Nd doped sample also (Fig. 9b). The estimated band gap for Nd doped sample is ~3.6 eV. So the shift is not due to the incorporation of Cr ion rather it is the effect of Nd ions substituting yttrium ions. In fact estimated band gap for all the Nd-Cr co-doped samples grown in different oxygen environment is almost similar, ranging between 3.55-3.6 eV. By theoretical calculation it has been argued that the contribution of 4f electrons either to the valence or conduction band could lead to a reduction of Eg for ortho-vanadates.23 Y3+ has no f electrons whereas Nd3+ cations have partially filled 4f orbitals which may lead to the observed red shift in the band gap energy.


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Figure 9. Estimation of band gap energy using Tauc relation for (a) Nd:Cr:YVO4 crystal grown in 100% oxygen atmosphere, (b) Nd:YVO4.


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CONCLUSIONS Single crystals of 1.4% Cr and 0.7% Nd co-doped yttrium ortho-vanadate were grown by optical floating zone technique in different oxygen atmosphere. Transmission measurement shows broad absorption bands due to Cr5+ along with sharp absorption lines of Nd3+ ions. There is a broad absorption band near 1100 nm (2A1→ 2B2 transition) that enables the crystal to be a self-Q-switched laser matrix. The absorption coefficient for this band is relatively more compared to that reported for the crystal grown by Cz method as the crystal grown by OFZ technique contains lesser oxygen vacancy. Further on annealing under continuous flow of oxygen 2A1→ 2B2 transition band was found to increase signifying the increase in the number of penta-valent state Cr ions due to compensation of oxygen vacancy in the lattice. The absorption of 2A1→ 2B2 transition further increased on increasing the oxygen content in the growth ambience. Hence for self Q-switching application bulk growth of this crystal by OFZ technique facilitates the maximum number of Cr ions to attain the desired penta-valent state. Annealing the crystals grown by Cz technique in oxygen ambience also resulted in the increase of the number of Cr ions in +5 state but being a solid state diffusion process it is restricted to small thickness. The observed red shift in the band-gap energy is due to the incorporation Nd in the lattice whereas Cr does not influence the band-gap energy.


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Growth of Nd:Cr:YVO4 Single Crystals by OFZ Technique under Different Oxygen Partial Pressure to Control the Oxidation State of Chromium Indranil Bhaumik, Sarveswaran Ganesamoorthy, Rajeev Bhatt, Amit Saxena, Ashwani Kumar Karnal and Pradeep Kumar Gupta Single crystals of Nd:Cr:YVO4 have been grown by optical floating zone (OFZ) technique. The investigations reveal that the bulk growth of the crystals by OFZ method results in an increase in the oxidation state of Cr to desired pentavalent state (responsible for self Q-switching) by increasing the oxygen content in the growth ambience, which is not possible by Czochralski technique.


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Growth of Nd:Cr:YVO4 Single Crystals by the Optical Floating Zone

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