Photoluminescence in Chemical Vapor Deposited ZnS - OSA Publishing

Photoluminescence in Chemical Vapor Deposited ZnS: insight into electronic defects John S. McCloy,1,2,* Barrett G. Potter3,4 1

Glass and Materials Science, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, WA, USA 2 School of Mechanical and Materials Engineering, Washington State University, Pullman, WA, USA 3 Department of Materials Science and Engineering, University of Arizona, Tucson, AZ, USA 4 Optical Science Center, University of Arizona, Tucson, AZ, USA *[email protected]

Abstract: Photoluminescence spectra taken from chemical vapor deposited (CVD) ZnS are shown to exhibit sub-band-gap emission bands characteristic of isoelectronic oxygen defects. The emission spectra vary spatially with position and orientation with respect to the major axis of CVD growth. These data suggest that a complex set of defects exist in the band gap of CVD ZnS whose structural nature is highly dependent upon local deposition and growth conditions, contributing to inherent heterogeneity in optical behavior throughout the material. ©2013 Optical Society of America OCIS codes: (160.2540) Fluorescent and luminescent materials; (300.6280) Spectroscopy, fluorescence and luminescence; (260.3060) Infrared.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

D. C. Harris, “History of the development of hot-pressed and chemical-vapor-deposited zinc sulfide and zinc selenide in the United States,” Proc. SPIE 6545, 654502 (2007). D. C. Reynolds and S. J. Czyzak, “Single synthetic zinc sulfide crystals,” Phys. Rev. 79(3), 543–544 (1950). W. M. Yen, S. Shionoya, and H. Yamamoto, eds., Phosphor Handbook, 2nd ed. (CRC Press, 2007). J. S. McCloy and R. W. Tustison, Chemical Vapor Deposited Zinc Sulfide (SPIE Press, 2013). J. McCloy, R. Korenstein, and B. Zelinski, “Effects of temperature, pressure, and metal promoter on the recrystallized structure and optical transmission of chemical vapor deposited zinc sulfide,” J. Am. Ceram. Soc. 92(8), 1725–1731 (2009). J. McCloy, E. Fest, R. Korenstein, and W. H. Poisl, “Anisotropy in structural and optical properties of chemical vapor deposited ZnS,” Proc. SPIE, 8016, 801601 (2011). A. F. Shchurov, E. M. Gavrishchuk, V. B. Ikonnikov, E. V. Yashina, A. N. Sysoev, and D. N. Shevarenkov, “Effect of hot isostatic pressing on the elastic and optical properties of polycrystalline CVD ZnS,” Inorg. Mater. 40(4), 336–339 (2004). E. Karaksina, V. Ikonnikov, and E. Gavrishchuk, “Recrystallization behavior of ZnS during hot isostatic pressing,” Inorg. Mater. 43(5), 452–454 (2007). D. C. Harris, M. Baronowski, L. Henneman, L. LaCroix, C. Wilson, S. Kurzius, B. Burns, K. Kitagawa, J. Gembarovic, S. M. Goodrich, C. Staats, and J. J. Mecholsky, “Thermal, structural, and optical properties of Cleartran multispectral zinc sulfide,” Opt. Eng. 47(11), 114001 (2008). K. Era, S. Shionoya, and Y. Washizawa, “Mechanism of broad-band luminescences in ZnS phosphors–I. Spectrum shift during decay and with excitation intensity,” J. Phys. Chem. Solids 29(10), 1827–1841 (1968). Y. Uehara, “Electronic structure of luminescence center of ZnS:Ag phosphors,” J. Chem. Phys. 62(8), 2982– 2994 (1975). R. H. Page, K. I. Schaffers, L. D. DeLoach, G. D. Wilke, and F. D. Patel, J. John B. Tassano, S. A. Payne, W. F. Krupke, K.-T. Chen, and A. Burger, “Cr2+ doped zinc chalcogenides as efficient, widely tunable mid-infrared lasers,” IEEE J. Quantum Electron. 33(4), 609–619 (1997). W. M. Yen and M. J. Weber, Inorganic Phosphors: Compositions, Preparation and Optical Properties (CRC Press, 2004). A. K. Collins, R. L. Taylor, X. Zhang, and B. D. Bartolo, “Optical characterization of polycrystalline ZnS produced via chemical vapor deposition,” Electrochemical Society Proceedings 90–12, 626–633 (1990). N. K. Morozova, I. A. Karetnikov, K. V. Golub, E. M. Gavrishchuk, E. V. Yashina, V. G. Plotnichenko, and V. G. Galstyan, “Pressure and temperature effects on point-defect equilibria and band gap of ZnS,” Inorg. Mater. 40(11), 1138–1145 (2004). K. L. Lewis, G. S. Arthur, and S. A. Banyard, “Hydrogen-related defects in vapour-deposited zinc sulphide,” J. Cryst. Growth 66(1), 125–136 (1984). X. Fang, Y. Bando, U. K. Gautam, T. Zhai, H. Zeng, X. Xu, M. Liao, and D. Golberg, “ZnO and ZnS Nanostructures: Ultraviolet-light emitters, lasers, and sensors,” Crit. Rev. Solid State Mater. Sci. 34(3–4), 190– 223 (2009).

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Received 23 May 2013; revised 1 Aug 2013; accepted 1 Aug 2013; published 9 Aug 2013

(C) 2013 OSA 1 September 2013 | Vol. 3, No. 9 | DOI:10.1364/OME.3.001273 | OPTICAL MATERIALS EXPRESS 1273

18. M. Aven and J. S. Prener, eds., Physics and Chemistry of II–VI Compounds (North-Holland & John Wiley and Sons, 1967). 19. N. K. Morozova, V. A. Kuznetsov, and M. V. Fok, Sul’fid tsinka. Poluchenie i opticheskie svoistva [Zinc Sulfide: Preparation and Optical Properties] (Nauka, 1987). 20. N. K. Morozova, I. A. Karetnikov, V. V. Blinov, and E. M. Gavrishchuk, “A study of luminescence centers related to copper and oxygen in ZnSe,” Sov. Phys.-Semicond. 35(1), 24–32 (2001). 21. N. K. Morozova, I. A. Karetnikov, V. G. Plotnichenko, E. M. Gavrishchuk, E. V. Yashina, and V. B. Ikonnikov, “Transformation of Luminescence Centers in CVD ZnS Films subject to a high hydrostatic pressure,” Sov. Phys.-Semicond. 38(1), 36–41 (2004). 22. N. K. Morozova, D. A. Mideros, V. G. Galstyan, and E. M. Gavrishchuk, “Specific features of luminescence spectra of ZnS:O and ZnS:Cu(O) crystals in the context of the band anticrossing theory,” Sov. Phys.-Semicond. 42(9), 1023–1029 (2008). 23. C. A. Klein and R. N. Donadio, “Infrared-active phonons in cubic zinc sulfide,” J. Appl. Phys. 51(1), 797–800 (1980). 24. H. A. Qiao, K. A. Lipschultz, N. C. Anheier, and J. S. McCloy, “Rapid assessment of mid-infrared refractive index anisotropy using a prism coupler: chemical vapor deposited ZnS,” Opt. Lett. 37(9), 1403–1405 (2012). 25. T. Zscheckel, W. Wisniewski, and C. Rüssel, “Microstructure and texture of polycrystalline CVD-ZnS analyzed via EBSD,” Adv. Funct. Mater. 22(23), 4969–4974 (2012). 26. J. McCloy, W. Wolf, E. Wimmer, and B. Zelinski, “Impact of hydrogen and oxygen defects on the lattice parameter of chemical vapor deposited zinc sulfide,” J. Appl. Phys. 113(2), 023706 (2013). 27. F. A. Kroeger and H. J. Vink, “The origin of the fluorescence in self-activated ZnS, CdS, and ZnO,” J. Chem. Phys. 22(2), 250–252 (1954).

1. Introduction Zinc sulfide has been known as an infrared-transmitting material since 1950 [1,2] and has been used as a phosphor for over one hundred years [3], for applications ranging from x-ray screens to cathode ray tube phosphors. Polycrystalline ZnS produced by chemical vapor deposition (CVD) was developed in the 1970s and has internationally become the most important long-wave infrared (LWIR, 8-12 μm) transmitting ceramic material for military systems due to its unique combination of thermal, mechanical, and optical properties [4]. Commercially produced CVD ZnS is yellow and optically scattering in the visible and midwave infrared (3-5 μm), and shows significant anisotropy in microstructure as-deposited, with elongated grains in the growth direction [5,6]. CVD ZnS recrystallizes and is converted to “water clear” “multispectral” (broad-band-transmitting from visible to LWIR) ZnS by postdeposition hot isostatic pressing (HIP), often in the presence of a metal like platinum [5,7,8]. HIP produces profound microstructural changes in CVD ZnS that result in increased grain size, thermal conductivity, elastic modulus, and visible and near-infrared transmission, along with decreased fracture strength and hardness [6,9]. Due to its large band gap of ~3.7 eV, ZnS is a very effective medium for phosphors (e.g., ZnS:Cu) [10], α-particle scintillators (e.g., ZnS:Ag) [11] and lasers (e.g. ZnS:Cr) [12]. Much is known about the luminescence of mass-produced ZnS for powder phosphors [13], but surprisingly little has been published on the electronic defects in CVD ZnS (with a few notable exceptions [14–16]), which is produced as bulk optics for its infrared transparency. Much recent work on luminescence in ZnS has focused on ZnS nanostructures, often favoring the hexagonal polymorph [17]. The most important kind of luminescence in ZnS is known as deep-center luminescence, where “deep” refers to the location of the defect energy levels within the forbidden energy gap. The term “center” refers to the fact that photoexcited carrier recombination occurs at spatially localized structural complexes typically encompassing multiple species within a radius of second or third nearest neighbor in the ZnS atomic lattice. The most commercially important ZnS phosphors use transition metal ion dopants to produce these deep levels, though rare earths are occasionally used as well [3]. Transition metal ion energy levels in ZnS and other II-VI semiconductor hosts have been well-studied, although the exact mechanisms for some luminescence behaviors have been hotly debated [18]. 2. Experimental setup A set of materials from a single CVD run of ZnS from a commercial source (DOW Chemical) were investigated to explore orientational effects on optical properties. Specimens consisted of material segments cut from a single, thick CVD deposit core. This 1” thick core represented

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approximately 500 hours (~21 days) of CVD growth from the first material deposited on the substrate (“mandrel” surface) to the last material deposited (“growth” surface). Individual, rectangular specimens with nominal dimensions of 12mm x 2mm x 1mm, optically polished on the large face, were cut from the core samples along two orientations (i.e., perpendicular and parallel to the growth direction) and at three points along the growth direction (mandrel, middle, and growth surfaces). Samples cut so that the largest face was in the plane of the growth direction were denoted “S,” while those where the largest face was perpendicular to the growth direction (representative of typical windows of CVD ZnS for many infrared applications) were denoted “P.” Additionally, both S and P samples were cut from the mandrel side (denoted C), middle (denoted B), and growth side (denoted A). Thus there were six possible samples representing two orientations of three positions in the core (see Fig. 1). Additionally, legacy CVD and HIP ZnS samples produced by Raytheon were investigated as well, with unknown orientations with respect to the growth axis. Photoluminescence (PL) was performed at both room temperature (RT, ~300 K) and at a sample temperature of approximately10 K (specimens denoted: LT) using a closed loop helium cryogenic cooler with optical access. Optical excitation at 248 nm pulsed KrF excimer laser (0.45 mJ/cm2/pulse, 800 ps pulse length 5 Hz repetition rate) for all measurements, with the laser oriented perpendicular to the large sample face. Standard integration time for the KrF excited PL was 5 – 10 seconds (dependent upon signal level) with the signal averaged over ten scans. The luminescence was analyzed using a 0.15 meter monochrometer with a backthinned Peltier-cooled CCD Si array equipped with a long pass filter to remove the excitation radiation. Spectra were collected over a wavelength range from 250 nm – 750 nm for the oriented samples, and 250 – 1000 nm for the unoriented samples. The larger wavelength range was initially chosen to assess the presence any short-wave infrared luminescence. Accuracy of features is estimated at ± 0.2 nm for sharp peaks and ± 2 nm for broad features.

Fig. 1. Schematic of orientations and microstructure of measured samples.

3. Results and discussion Oxygen-related defect complexes have been discussed as being the most important deep centers for luminescence in nominally undoped ZnS [19]. The defect complex is described as an isoelectronic oxygen defect (OS) locally associated with a zinc interstitial (Zni) and zinc vacancy (VZn), which represents a shift of the position of the zinc atom due to lattice strain compensation near the oxygen substitutional atom [20]. This acceptor defect complex can take three charge states, which are named SA(I), {OS-Zni•-VZn′′}′, SAL(II), {OS-Zni••-VZn′′}x, and (III), {OS-Zni••-VZn′}•, where SA indicates “self-activated” and SAL is SA luminescence, with characteristic emission bands. A summary of wavelengths of noted spectral features in each sample is given in Table 1. Figure 2(a) shows PL spectra of unoriented Raytheon-produced CVD ZnS. Low temperature spectra indicate a sharp peak at 327.2 nm which could be assigned to the SA donor-bound #191002 - $15.00 USD



exciton I2 (Zni•) [21]. The LT peak at 448 nm can therefore be assigned to the donor-acceptor pair SA(I) center [20] present in materials with an excess of metal. In the RT spectrum, the main band is at 495 nm and the exciton-related peak at 337 nm. Based on estimated binding energies for excitons, the 337 nm should be assigned to a slightly red-shifted free exciton (FE) [21]. We could also assign this to a blue-shifted I1(SAL) acceptor-bound exciton, linked to the charge state of the oxygen complex that dominates stoichiometric compositions [21] and those with very low oxygen concentration [22]. However, there are other indications suggesting that the 337 nm is not related to the SAL center. This specimen lacks the SAL conduction band to acceptor transition expected at 355 – 370 nm in the LT spectrum. Additionally, the binding energy for the SAL center is reported at 24.5 meV [21] which is very close to RT (~25 meV) suggesting that this exciton should be ionized and not bound to a SAL defect at RT. The alternative explanation for the 337 nm band (RT), which we favor, is that it is the FE band, red-shifted (IFE,O) from its usual position at 336.2 nm (RT) [21]. Red shifts of the RT exciton band to wavelength positions up to 342 nm have been reported due to the presence of dissolved oxygen and the associated change in the band gap [15]. The longer wavelength hump at 353 nm (RT) is likely part of a manifold of phonon replicas of IFE,O. The longitudinal optical (LO) phonon energy of ZnS is 0.043 eV (350 cm−1) [23], so phonon replicas of 337 nm will be at 340 nm (IFE,O-LO), 344 nm (IFE,O-2LO), 348 nm (IFE,O-3LO), and 353 (IFE,O-4LO). Figure 2(b) shows the spectra collected for multispectral (HIP) ZnS. The FE is evident at low temperatures (326.8 nm) and at room temperature (336.8 nm). Weak emission in the ultraviolet in the LT spectrum may indicate the SAL(II) (359 nm) complex in local stoichiometric regions, while emission in the visible may be related to other oxygen-related centers of SA(I) (459 nm) and (III) (525 nm) [20]. The second-order diffracted line associated with the exciton is observable in the LT spectrum at 654nm ( = 2x327) and the third-order line is also observed at 986nm (close to 3x327 = 981nm). Unlike the standard CVD ZnS, where emission intensities are comparable at RT and LT, the HIP ZnS shows a dramatic increase in emission intensity when going to LT.

Fig. 2. Photoluminescence at room (300 K) and low (10 K) temperatures in (a) unoriented Raytheon CVD ZnS (b) unoriented HIP ZnS. Inset whole spectrum, main figure exciton region

Figure 3(a) shows the LT PL and Fig. 3(b) the RT PL spectra of the oriented CVD samples. The most striking comparison is that both the RT and LT spectra of the growth-side sample, S orientation, is more than an order of magnitude more intensely luminescing than all other samples. All the growth side measurements (RT and LT in orientations S and P) show a completely quenched exciton and a single broad intense PL centered in the green. In all middle and mandrel-side measurements (RT and LT), the luminescence in the P orientation was higher than the S, and the exciton is evident, contrary to the measurements on the growth side. For the mandrel-side measurements, RT and LT measurements have nearly the same intensity for each pair (S and P) except for the exciton which is very strong in LT and very

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weak in RT. The peak visible emission is slightly blue-shifted in the RT measurements of each pair. For the middle-core samples, the visible PL intensity is ~3x higher for the LT than the RT measurements. S and P orientated samples give very similar spectra, with excitons ~327 (LT) and ~337 (RT) similar to other samples and two visible lobes ~450 and ~500 nm (LT), corresponding to the SA(I) and (III) defect complexes, respectively. The LT spectra of the middle-core samples show clear phonon replicas of the 327 nm exciton at 331 nm (IFE,OLO) and 338 (probably superposition of 335 nm (IFE,O-2LO) and 339 nm (IFE,O-3LO). Table 1. Spectral features in PL of CVD ZnS samples. Sample

Orientation

CVD

Unknown, P

337, 353, 495

RT spectral Positions (nm)

LT spectral Positions (nm) 327.2, 331, 448, 772

HIP

Unknown, P

336.8, 434

326.8, 359, 459, 495, 525, 564

CVD

Mandrel, P

335.9, 337.8, 388, 412, 436

326.3, 336.8, 446, 495

CVD

Mandrel, S

383, 407, 418, 498

326.7, 449, 492

CVD

Middle, P

335.9, 414, 479, 548

326.8, 330.6, 338, 351, 386, 450, 496 326.8, 331.1, 338, 444, 499

CVD

Middle, S

336.8, 457, 487

CVD

Growth, P

517

514

CVD

Growth, S

498 (v intense)

498, 518 (both v intense)

Fig. 3. Photoluminescence in core: (a) low temperature (b) room temperature. Only the “growth S” spectra correspond to the righthand y-axes. Asterisk (*) indicates the 2nd and 3rd order diffracted line of the exciton.

These results indicate a substantial complexity in the sub-band-gap defect states of CVD ZnS even within a single ingot of material. Visible and infrared refractive indices measured for these different samples showed no significant differences [24] despite different texturing of the hexagonal stacking and lattice parameter along the growth direction [6]. Recent work with electron backscatter diffraction [25] has shown that the mandrel side (first to deposit) consists of grains