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 mid-wave 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 post-deposition 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 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.

 

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

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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 back-thinned 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.

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 (O_S) locally associated with a zinc interstitial (Zn_i) and zinc vacancy (V_Zn), 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), {O_S-Zn_i^•-V_Zn′′}′, SAL(II), {O_S-Zn_i^••-V_Zn′′}^x, and (III), {O_S-Zn_i^••-V_Zn′}^•, 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 exciton I₂ (Zn_i^•) [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 (I_FE,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 I_FE,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 (I_FE,O-LO), 344 nm (I_FE,O-2LO), 348 nm (I_FE,O-3LO), and 353 (I_FE,O-4LO).

Table 1. Spectral features in PL of CVD ZnS samples.

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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

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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.

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 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 (I_FE,O-LO) and 338 (probably superposition of 335 nm (I_FE,O-2LO) and 339 nm (I_FE,O-3LO).

 

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 2^nd and 3^rd order diffracted line of the exciton.

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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 <10 μm with a statistically random orientation, while the middle portion of CVD ingots has elongated grains 100’s of μm long containing lamellae within them, and the growth side consists of elongated grains <25 μm oriented <001> direction parallel to the growth direction.

This PL data highlights the rich and complex defect structure in CVD and HIP CVD ZnS and strengthens the claim of oxygen incorporation. It also suggests the possibility of Zn-rich ZnS (associated with oxygen), since the ~445 nm donor (Zn_i^•)-acceptor ({O_S-Zn_i^•-V_Zn′′}′)-pair blue band (SA) is almost always dominant in the measured data here. Only HIP ZnS and possibly middle-P show significant luminescence in the SAL(II) region (355 – 370 nm) which is indicative of stoichiometric material. However, both the middle-core and the HIP ZnS have large grains, and the crystallographic perfection could be indicated by the strength of the exciton relative to the deep center, which is largest in the HIP sample. However, HIP ZnS still has a substantial band that seems to be composed of the blue and green bands, and so therefore it may be more stoichiometric than the other materials but still has regions of Zn-richness (blue band) and some of S-richness (green band). Note that in this defect model, ZnS always has some oxygen incorporated and the local defect structure around oxygen (whether this is zinc interstitials or substitutional dopant ions) which will dominate the luminescence spectrum. It is known from chemical measurements on these samples that there are more than sufficient oxygen defects (~10²⁰ cm⁻³) [26].

It has been shown by Morozova et al [20] that the blue and green bands are related to the oxygen isoelectronic defect, and that interstitial zinc can play the same role as interstitial copper in this complex with very little spectral change. Kroeger and Vink [27] postulated that the blue self-activated luminescence in ZnS was due to V_S^• or Zni_i^•, both of which could be present in Zn-rich material and which could form the co-activator (donor) in the luminescence complex. Lewis et al [16] also performed luminescence measurements of their CVD grown ZnS excited by a mercury lamp and filters making the excitation wavelength 365 nm. They observe a local variation of this PL in samples taken from different parts of the growth chamber exhibiting different levels of visible and infrared (6 μm) absorption, but which do not have measurable differences in copper concentration. The variation in concentration of the zinc hydride defect responsible for the 6 μm absorption is used to explain the difference in intensity of green (2.39 eV, 520 nm) emission through a charge-transfer quenching process. Elsewhere they state that in cathodoluminescence studies where samples are also imaged, the green emissions have been shown to either increase or decrease at grain boundaries. We offer an alternative model to explain the luminescence data of Lewis et al [16]. Rather than having the copper defect be responsible for the green luminescence, it is possible that in fact an oxygen complex is responsible, such as (III) {O_S-Zn_i^••-V_Zn′}^•. It has been shown that isoelectronic complexes associated with native zinc defects are spectrally very close to those with copper defects [20]. It is likely that the concentration of oxygen defects would vary with the concentration of zinc hydride defects, since both occupy a sulfur site [26].

7. Summary

It is apparent from this short discussion that PL provides an extremely complicated picture of the electronic defects in CVD ZnS. There is considerable variation in the luminescence observed in different positions and orientations along the CVD growth direction. This is presumably due to the concentration of quenching impurities (by nonradiative mechanisms or charge transfer) which increases at the growth side. The HIP process produces multispectral ZnS which may be more stoichiometric than the CVD ZnS, as evidenced by the emission energy of the SAL band in the ultraviolet. The visible luminescence can be resolved into other peaks suggesting underlying structure to the broad-band luminescence.