Optical and EPR spectroscopy of Zn:Cr:ZnSe and Zn:Fe:ZnSe crystals

Optical Materials 37 (2014) 262–266

Contents lists available at ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

Optical and EPR spectroscopy of Zn:Cr:ZnSe and Zn:Fe:ZnSe crystals V.V. Fedorov ⇑, T. Konak, J. Dashdorj, M.E. Zvanut, S.B. Mirov Physics Department, University of Alabama at Birmingham, 1300 University Blvd., Birmingham, AL 35294, United States

a r t i c l e

i n f o

Article history: Received 17 March 2014 Received in revised form 3 June 2014 Accepted 4 June 2014 Available online 15 July 2014 Keywords: Transition-metal-doped materials Laser materials Transition metal solid-state lasers

a b s t r a c t Optical and EPR characterization of Cr and Fe doped ZnSe crystals annealed in Zn vapor revealed a strong bleaching of the divalent state of transition metal ions. Photo induced EPR kinetics were studied in 20–80 K temperature range. Analysis of time-dependent data reveals Cr1+ signal rise time decreases with increasing temperature. The non-exponential decay of Cr1+ concentration were analyzed using Augertype recombination process. The photoluminescence quantum yield of Cr2+ ions at 5E(D) ? 5T2(D) mid-IR transition excited via chromium ionization process was measured to be close to 100%. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Transition metal (TM) doped II–VI binary (e.g., ZnSe, ZnS, CdSe, CdS, ZnTe) and ternary (e.g., CdMnTe, CdZnTe, ZnSSe) chalcogenides are arguably the most effective gain media for broadly tunable high power/energy middle-infrared (mid-IR) lasers [1–3]. For example, optically pumped room temperature (RT) lasers based on Cr:ZnS, Cr:ZnSe, Cr:Cd1xMnxTe, Cr:CdSe, and Fe:ZnSe crystals providing access to the 2–6 lm spectral region with high (up to 70%) efficiency, multi-Watt-level CW output power (>23 W), tunability in excess of 1000 nm, narrow spectral line width (<700 kHz), high output pulse energy (>1 J), and high peak power in mode-locked operation mode (>1 GW at 300 fs) have been reported (see [1–3] and references therein). In addition to effective RT mid-IR lasing TM doped II–VI media, being wide band gap semiconductors, hold potential for direct electrical excitation [4–8]. Zinc vacancies (VZn), zinc interstitials (Zni), (VZn) – (Zni) Frenkel pairs as well as charge states of donor and acceptor impurities play important role and essentially define the electrical properties of II–VI crystals. In this paper, Cr and Fe doped ZnSe polycrystals annealed in Znvapor were studied. It was found that ZnSe annealing in Zn-vapor results in a strong bleaching of the TM divalent state. Optical and electron paramagnetic resonance (EPR) spectroscopy characterizations of Cr:ZnSe and Fe:ZnSe crystals before and after annealing were used to identify possible charge state of active ions after annealing. Mid-IR Cr2+ 5E(D) ? 5T2(D) photoluminescence excited via Cr2+ + hmvis ? Cr1+ + h+ ? Cr2+ ionization process was ⇑ Corresponding author. Tel.: +1 205 934 5318; fax: +1 205 934 8042. E-mail address: [email protected] (V.V. Fedorov). http://dx.doi.org/10.1016/j.optmat.2014.06.004 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.

demonstrated earlier [4,9], however, quantum yield value of mid-IR photoluminescence under this excitation was not reported yet. This excitation mechanism is important not only as a new possible route for optical pumping of tunable mid-IR lasers, but it is also essential for effective electrical excitation of Cr2+ ions. Therefore, quantitative comparison of the mid-IR photoluminescence under direct and ionization excitations was one the goals of current research. 2. Results and discussions Cr:ZnSe and Fe:ZnSe samples were prepared by post-growth thermal diffusion of chromium and iron impurities in polycrystalline ZnSe CVD grown by II–VI, Inc. Chromium or iron thin films (50–150 nm) were deposited on the two largest crystal surfaces of the samples (10  5  2.5 mm3) via thermal evaporation (see [1] and references therein). The samples were sealed in quartz ampoules under high vacuum (<104 Torr), placed in a furnace, and heated up to 900–1000 °C for a 7 days. After thermal diffusion Cr:ZnSe and Fe:ZnSe crystals were removed from the ampoules, cut and polished to a size of 10  5  2 mm3. The workpieces doped with 7  1018 cm3 and 2  1019 cm3 chromium and iron, respectively, were selected for this study. The concentrations of Cr2+ and Fe2+ ions in the samples were estimated using values of the absorption cross-section for the 5E(D) ? 5T2(D) transition, to be 1.1  1018 cm2 at 1.75 lm and 0.6  1018 cm2 at 2.698 lm for chromium and iron ions, respectively [2]. 5  5  2 mm3 samples were cut from these doped workpieces, polished, and cleaned with acetone and methanol. Each sample was sealed with 0.2 g of Zn in quartz ampoules under vacuum pressure of 102 Torr and annealed at 1000 °C for 2 days.

263

V.V. Fedorov et al. / Optical Materials 37 (2014) 262–266

-1

12

(A) Cr:ZnSe

10 8

i

6 4 2 0

states of Cr2+ ions, one can estimate the excitation rate as sex1 = 1/ 4 ls1, which is faster than the relaxation rate from 5E(D) state. As one can see from Fig. 1(A), the annealing of the Cr:ZnSe crystal in Zn-vapor results in elimination of 1.7 lm absorption band corresponding to the 5T2 ? 5E transition of Cr2+ ions in ZnSe. In addition, the strong absorption band near the bang gap transition at about 500 nm also vanished. Thus, total Cr2+ concentration change was about 7  1018 cm3 during annealing. Cr4+ ions in crystals with tetrahedral crystal site symmetry have strong absorption bands around 600–800 nm due to 3A2 ? 3T1(F) transition and the weaker absorption band in the range of 900–1150 nm due to 3 A2 ? 3T2(F) transition [11,12]. Cr3+ ions also have characteristic 4 A2 ? 4T2 absorption band around 500–700 nm [13]. Therefore, the absence of new absorption bands in the visible-NIR spectral range of the Zn:Cr:ZnSe crystals is a good indication that Cr3+ and Cr4+ ions are not formed in Cr:ZnSe crystals during their annealing in Zn vapor. Spectra of Fe:ZnSe samples before and after annealing in Zn vapor are also depicted in Fig. 1(B). The absorption band centered at 3100 nm is due to 5E ? 5T2 transition of the Fe2+ ions. After annealing, Fe2+ ions still could be seen in the crystals. However, the absorption coefficient of Fe2+ ions at 5E ? 5T2 transition changed from 20 to 14 cm1, corresponding to Fe2+ concentration change of 6.3  1018 cm3. The annealing in the Zn vapor also results in decreasing of the charge transfer (Fe1+ + hmvis ? Fe2+ + e) band around 500 nm. EPR spectra of Cr1+ and Fe3+ ions in II–VI semiconductors have been studied for many decades and are well documented [10,14– 20]. We use the results of these studies to examine the valence states of iron and chromium ions not easily accessible by optical methods. EPR spectra of the samples were recorded at X-band (9.4 GHz) between 3.5 and 160 K. All of the charge centers were identified through EPR parameters such as the principal g-values and hyperfine coupling constants as well as the cubic splitting parameters as described in many Refs. [16,17,21,22]. The absolute number of centers was determined by double integration of the spectra and comparison with a calibrated Si:P powder. For photo-EPR measurements, a 250 W quartz-tungsten-halogen lamp in combination with a grating monochromator was used for excitation. Here, the samples were illuminated through the slits of the EPR cavity for 2–5 min. A constant photon flux of 2.8±1.2  1014/(m2 s) for all selected wavelengths was obtained with neutral density filters. An EPR spectrum reflects a specific charge state, thus changes in signal intensity with illumination are interpreted as removal or capture of an electron onto the specific ion. Time-dependent photo-EPR measurements were measured with a resolution of 10 ms by setting the magnetic field at the peak of the intensity and recording the intensity as a function of time during and after illumination. 1.5  1.5  5 mm3 pieces were cut from the polished 10  5  2 mm3 pieces for EPR measurements.

Absortption coeffitient, cm

Absorption coefitient, cm

-1

The room temperature absorption spectra were measured by UV–VIS–NIR and FTIR spectrophotometers over visible – IR spectral range. Spectra of Cr:ZnSe samples before and after annealing in Zn vapor are depicted in Fig. 1(A). In all II–VI semiconductor hosts, laser active Cr2+ (3d4) and Fe2+ (3d6) ions predominantly occupy cation lattice position with Td symmetry. Only trace amount of Cr1+ or Fe3+ valence state could be detected by EPR spectroscopy. The absorption band spanning over 1500–2100 nm spectral range is due to Cr2+, 5T2(D) ? 5E(D) transition and is clearly seen for the Cr:ZnSe crystal before annealing in Zn vapor. Direct excitation into this band reveals mid-IR photoluminescence between 1.8 and 3.3 lm (Fig. 2(i)). This spectrum was not corrected for the sensitivity of the spectrometer and an additional 2 lm filter was used to eliminate pump radiation at 1532 nm. The photo-ionization band is also clearly seen in the absorption spectrum near the band gap (500 nm) transition. Previously, it was demonstrated that excitation into this band results in a Cr2+ ionization to the Cr1+ valence state and hole formation in the valence band (Cr2+ + hmvis ? Cr1+ + h+) as shown in Fig. 4(b-i) [10]. Processes including change of the valence state of chromium ions could be very attractive as possible mechanisms for electrically pumped Cr:ZnSe laser systems. Two parameters are important in these processes. First of all, the efficiency of excitation to 5E chromium energy level due to the relaxation processes Cr1+ + h+ ? Cr2+. Second parameter is excitation rate, sex1, which should be faster than the decay rate, s21 from 5E(D) state. In our experiments we compared photoluminescence spectrum under direct CW excitation using 1.56 lm fiber laser and 532 nm radiation from the second harmonic of Nd:YAG laser. Green laser excitation results in Cr1+ formation, followed by a relaxation to the Cr2+ valence state. Fig. 2(i and ii) depicts photoluminescence spectra for both excitations. In these experiments, both excitation lasers were focused to the same spot of the 1 mm thick Cr:ZnSe sample. The switch between two excitation wavelengths does not result in any changes in the registration system. The results shown in Fig. 2 were normalized to the absorbed photon flux. As one can see from the figure, both IR spectra and signal amplitudes are similar for both excitations. Therefore quantum yield under green excitation (532 nm) is close to 1 by taking into account that the quantum yield of Cr2+ mid-IR luminescence under direct excitation is found to be 1 from the previous work [1,2]. To estimate excitation rate under green excitation the mid-IR PL kinetic measurements were performed at room temperature under optical pumping by pulsed radiation 532 and 1560 nm with pulse duration about 5 ns. The time dependence of the mid-IR PL emission intensity is depicted in Fig. 2(iii and iv). It demonstrates photoluminescence kinetics under intra-shell 1560 nm excitation with a fast rise time, followed by a single exponential decay with s2 = 6.1 ls. Photoluminescence kinetics under 532 nm excitation shows an initial growth, which reaches a peak at 5 ls. Using a single exponential approximation for the decay process from upper

ii 0.5

1.0

1.5

wavelength, μm

2.0

2.5

20

(B) Fe:ZnSe i

15 ii

10 5 0 0.4

0.6

0.8

1.0

3.0 4.0 5.0

6.0

wavelength, μm

Fig. 1. Absorption spectra of (A) Cr:ZnSe and (B) Fe:ZnSe before (i, dashed lines) and after (ii, solid lines) annealing in Zn vapor in the visible and mid-IR spectral ranges.

264

V.V. Fedorov et al. / Optical Materials 37 (2014) 262–266

Signal, ab.un.

Signal, ab.un.

i

1.8

2.0

2.2

2.4

2.6

2.8

3.0

B

iv

A

ii

3.2

iii

0

5

10

15

20

Tine, μs

wavelength, μm

Fig. 2. (A) Photoluminescence spectra of Cr:ZnSe crystal under 1560 nm (i – solid line), 532 nm (ii – dashed line) excitations normalized to the same number of absorbed photons and (B) kinetics of Cr:ZnSe mid-IR photoluminescence under 1560 nm (iv – solid line) and 532 nm (iii – dashed line) excitations.

The g-values and hyperfine constants were obtained by simulating the spectra and adjusting the EPR parameters until an adequate comparison to the measured spectrum was obtained. Typical EPR spectra obtained from Cr- and Fe-doped ZnSe measured at 40 K are shown in Fig. 3. For Cr1+ marked in the figure, a g-value of 2.0017 and hyperfine coupling constants of ACr = 14.2 G, A1 = 6.9 G, and A3 = 1.8 G were determined, similar to those in the literature [16,22]. Likewise, the measured EPR parameters for Fe3+, g = 2.0448 and AFe = 9.4 G, are close to the well-known reported values [22]. In addition to the intentionally introduced ions, trace amounts of Mn were found in all samples. Two of Mn2+ EPR sextet lines are indicated in Fig. 3. Table 1 shows the concentrations of Cr1+, Fe3+, Mn2+, Cr2+ and Fe2+ before and after annealing measured by EPR and optical absorption (OA) spectroscopy. The concentrations obtained from EPR for Cr1+ and Fe3+ are at least three orders of magnitude smaller than those extracted from the optical absorption of Cr2+ and Fe2+, verifying that the latter are the dominate charge states for the impurities both before and after annealing. The increase in Cr1+ and Mn2+ after annealing likely results from the impurities present in the Zn or furnace ambient. The photo-EPR data discussed next verify that their presence does not contribute to the optical absorption of the dominant Cr and Fe ions. Photo-induced EPR has been used to study transitions of chromium and iron ions in ZnSe crystals by several research groups. Godlewski and co-authors have shown that divalent iron and chromium ions in ZnSe have a ‘‘red threshold’’ near 1.9 eV [10]. The photo EPR results for Cr1+ (filled circles) obtained from the Cr-doped sample and Fe3+ (unfilled squares) obtained from the Fe-doped sample shown in Fig. 4(a) reveal thresholds expected for these ions in ZnSe [10,18,19]. The vertical axis represents the amount of the charge centers relative to the measured for that center in the dark. The data were obtained by collecting EPR spectra of the samples during illumination with different photon

Table 1 EPR and OA spectroscopy measured concentrations (1016 cm3) of impurities in ZnSe samples before and after annealing. Sample

Annealed

EPR

OA

Cr1+

Fe3+

Mn2+

Cr2+

Fe2+

Cr:ZnSe Zn:Cr:ZnSe

– 2 days@1000 °C

0.005 –

– –

0.26 1.3

700 <5

– –

Fe:ZnSe Zn:Fe:ZnSe

– 2 days@1000 °C

0.002 0.13

0.018 0.008

0.019 0.026

– –

2000 1300

energy, starting with the lowest energy. The width of the circles and squares reflect the uncertainty in photon energy associated with the bandwidths of the light source. These results, along with the analysis of the time dependence discussed below, suggest that the dominant transition process for Cr under 469 nm illumination is excitation of an electron from the valence band edge to the Cr2+ centers, producing Cr1+ (2.1 eV transition, Fig. 4(b-i)). Similarly, in Fe:ZnSe, the dominant process is excitation of an electron from the Fe2+ to conduction band edge (2.3 eV threshold, Fig. 4(b-iii)). The similarity of the thresholds to earlier work reinforces that the unintentional impurities do not affect the optical process of the Cr or Fe ions. To understand the kinetics of photoionization, the time dependence of the EPR intensity was monitored between 3.5 and 160 K. Examples of the time-dependence for the Cr1+ signal in the Crdoped sample obtained at 22 K (a) and 78 K (b) during illumination (ON) and after the light was removed (OFF) are shown in Fig. 5. The time dependence during illumination fits well the first-order kinetic model where capture of a valence band electron by Cr2+ creates Cr1+. The number of Cr1+ produced is given by a single exponential [23,24],

nCr1þ ðt Þ ¼ n1  n2 exp ðt=sR Þ

ð1Þ 1+

EPR signal, a.u.

Mn2+ i Fe3+ Cr1+

ii

x4 3250

3300

3350

3400

Magnetic field ,G Fig. 3. EPR spectra of Cr1+, Fe3+ and Mn2+ centers obtained from (a) Cr:ZnSe and (b) Fe:ZnSe polycrystals at 40 K.

where nCr1þ is the photo-induced Cr concentration, n1  n2 is the Cr1+ concentration in the dark and s is the signal rise time during the light illumination. Examination of the rise time for the data obtained at different temperatures shows that at T > 50 K, s decreases with increasing temperature, typical of behavior for recombination centers in semiconductors. The detailed temperature dependence is shown in Fig. 6(a). Kinetics of excited states of Cr+ ions under a low power lamp excitation and their population temperature dependencies could be described by a simple model for thermally activated radiationless transitions [33]:



s1 R ¼ W 0 þ W a exp 

 DE a ; kB T

ð2Þ

where DEa – activation energy, kB – Boltzmann constant, W0 and Wa are rate constants. The best fit to the experimental data was

265

V.V. Fedorov et al. / Optical Materials 37 (2014) 262–266

(b)

(a)

Fig. 4. (a) Spectral dependencies of the photoexcitation of Cr1+ (filled circles) and Fe3+ (unfilled circles) in Cr:ZnSe and Fe:ZnSe samples, respectively measured at 40 K. (b) Model for (i) photoionization of Cr2+ ? Cr1+ (ii) Cr1+ ? Cr2+ activated by recombination via Auger-type recombination process, and (iii) photoionization of Fe2+ ? Fe3+ transition mechanism in ZnSe crystal.

Fig. 5. Time-dependence of the Cr1+ EPR intensity obtained during and after exposure to 496 nm light for Cr: ZnSe sample at (a) 22 K and (b) 78 K.

B

A

W=1/τ, sec

-1

EPR signal,a.u.

iv T=22K

ii T=65K

T=45K iii

i T=78K

0

10 20 30 40

1000/T, K

150 200 250 300 -1

0

10

20

30 1/2

40

(Time) , sec

50

60

1/2

1+

Fig. 6. (a) Temperature dependence Cr rise times observed during 496 nm light illumination of Cr:ZnSe. The rise times were obtained from the best fit of Eq. (1) to the data. (b) Decay of the Cr+ EPR signal after exposure to 496 nm light for Cr:ZnSe crystal at 22, 45, 65, and 78 K, respectively. The dashed lines are fits to the data using Eq. (3).

obtained with parameters W0 = 1/35 s1 Wa = 2 s1 and DEa = 43 meV and is shown in Fig. 6a. Understanding the dynamic of the reverse process, Cr1+ ? Cr2+, is essential for optimization of electrical excitation of Cr2+ ions. However, attempts to use common defect-related charge transfer models with exponential decay failed to produce adequate fits to the data. Earlier, the Auger type recombination for Cr1+ ? Cr2+ process was discussed in [20,25]. Based on the results from anti-Stocks photoluminescence and EPR experiments, the authors proposed a model of energy transfer when hole re-trapping by Cr1+ results in the quasi-resonant ionization transition of deep acceptors (as illustrated in Fig. 3b-ii). The energy transfer rate in this process strongly depends on a distance and a type of interactions between Cr1+ ion (sensitizer) and deep acceptor (activator). Since first theoretical considerations by Förster [26] and Dexter [27], the macroscopic theory of the energy transfer process in doped materials has been

a subject of comprehensive studies by several research groups [28–31]. A detailed analysis of different energy transfer models is summarized in [32,33]. For the dipole–dipole interactions between the Cr1+ ions and deep acceptors, the decay of the Cr1+ concentration due to energy transfer process can be approximated by Eq. (3) [32,33]:

rffiffiffiffiffiffiffi  t t ; nCr1þ ¼ n3 exp  

ss

sDA

ð3Þ

where n3 is the Cr1+ concentration immediately after the light is turned off. First exponential term exp (t/ss) corresponds to intrinsic decay in absence of activators with constant ss. The second  the pffiffiffiffiffiffiffiffiffiffiffi ffi term exp  t=sDA corresponds to sensitizer-activator energy transfer process in the absence of sensitizer-sensitizer transfer process. The parameter sDA depends on the oscillator strengths and

266

V.V. Fedorov et al. / Optical Materials 37 (2014) 262–266

spectral overlap integral of the transitions in deep acceptors and Cr1+ ions as well as on the acceptor concentration. Fig. 5(b) shows pffiffi the dependence of the log ðnCr1þ Þ versus t measured at different temperatures after turning off the light. As one can see from the figure the plotted experimental data could be fitted by linear dependences. It means that Auger type recombination is dominant in comparison with single exponential decay within studied temperature and time scales. The parameter sDA reveals strong temperature dependence. It changes from 1842 s at 22 K to 12 s at 78 K. A strong increase of the energy transfer rate could be explained by an increase of the overlap integral due to spectral broadening and better overlap of the transition lines of Cr+ ions and acceptor with temperature increase. 3. Conclusions Optical, electrical and EPR characterization of chromium and iron doped ZnSe crystals annealed in Zn vapor demonstrate that annealing result in bleaching of the laser active Cr2+ and Fe2+ ions. The decreasing of the divalent states of the active ions cannot be explained by ionization of the TM to other valence states such as: Cr3+, Cr4+, Cr1+, and Fe3+. We believe that the annealing in the zinc vapor could result in purification of Cr:ZnSe and Fe:ZnSe crystal or formation of the chromium and iron neutral centers in the interstitial positions. The quantum yield of the mid-IR photoluminescence at 5E(D) ? 5T2(D) transition of the Cr2+ ions under ionization transition was measured to be 1. The rise-time of the room temperature mid-IR luminescence under this excitation was equal to 4 ls. It suggests that the ionization mechanism, Cr1+ + h+ ? Cr2+, could be effective for electrical pumping of the Cr:ZnSe solid state laser. Kinetics of the Cr1+ EPR signal decay in Cr:ZnSe support a model involving a secondary recombination pathway. Analysis suggests that the Cr1+ EPR signal decay is facilitated by interaction with a deep acceptor via Auger-type recombination process [10,25]. Annealing results in only partial suppression of Fe2+ active ions and it was accompanied by appearance of electro-conductivity, which is a promising route for fabrication of future electrically pumped Fe:ZnSe gain elements. The annealing of Cr or Fe doped ZnSe crystals in Zn vapor improves order in Zn sublattice and enables fabrication of Cr:ZnSe and Fe:ZnSe gain elements with undoped ends promising for high power applications. Acknowledgements The authors would like to acknowledge funding support from the AF Office of Scientific Research (Award No. FA9550-13-10234), and the National Science Foundation under grants EPS0814103 and CBET-1321551. References [1] S. Mirov, V. Fedorov, I. Moskalev, D. Martyshkin, C. Kim, Progress in Cr2+ and Fe2+ doped mid-IR laser materials, Laser Photon. Rev. 4 (2010) 21–41. [2] S. Mirov, V. Fedorov, I. Moskalev, M. Mirov, D. Martyshkin, Frontiers of midinfrared lasers based on transition metal doped II–VI semiconductors, J. Lumin. 133 (2013) 268–275. [3] P. Moulton, E. Slobodchikov, 1-GW-Peak-Power, Cr:ZnSe Laser, CLEO 2011 – Laser Applications to Photonic Applications, OSA Technical Digest (CD) (Optical Society of America, 2011), PDPA10. [4] V.V. Fedorov, A. Gallian, I. Moskalev, S.B. Mirov, En route to electrically pumped broadly tunable middle infrared lasers based on transition metal doped II–VI semiconductors, J. Lumin. 125 (2007) 184–195.

[5] J. Jaeck, R. Haidar, E. Rosencher, M. Caes, M. Tauvy, S. Collin, N. Bardou, J. Luc Pelouard, F. Pardo, P. Lemasson, Room-temperature electroluminescence in the mid-infrared (2–3 lm) from bulk chromium-doped ZnSe, Opt. Lett. 31 (2006) 3501–3503. [6] N.A. Vlasenko, P.F. Oleksenko, Z.L. Denisova, M.O. Mukhlyo, L.I. Veligura, Crrelated energy levels and mechanism of Cr2+ ion photorecharge in ZnSe:Cr, Phys. Stat. Solidi 245 (2008) 2550–2557. [7] N.A. Vlasenko, Z.L. Denisova, Ya.F. Kononets, L.I. Veligura, Yu.A. Tsyrkunov, Near-infrared-emitting ZnS: Er and ZnS(Se): Cr TFEL devices, J. Soc. Inf. Display 12 (2004) 179–182. [8] J. Jaeck, R. Haidar, F. Pardo, J.-L. Pelouard, E. Rosencher, Electrically enhanced infrared photoluminescence in Cr:ZnSe, Appl. Phys. Lett. 96 (2010) 211107. [9] L. Luke, V. V. Fedorov, I. S. Moskalev, A. Gallian, S. B. Mirov, Middle-infrared electroluminescence of n-type Cr doped ZnSe crystals, in: Proc. of SPIE Solid State Lasers XV: Technology And Devices, 6100(2006), 6100Y. [10] M. Surma, M. Godlewski, T. Surkova, Iron and chromium impurities in ZnSe as centers of nonradiative recombination, Phys. Rev. B 50 (1984) 8319–8324. [11] M.Y. Sharonov, A. Bykov, V. Petricˇevic´, R. Alfano, Cr4+-doped Li2CaSiO4 crystal: growth and spectroscopic properties, Opt. Commun. 231 (2004) 273–280. [12] K.V. Yumashev, N.V. Kuleshov, P.V. Prokoshin, A.M. Malyarevich, V.P. Mikhailov, Excited state absorption of Cr4+ ion in forsterite, Appl. Phys. Lett. 70 (1997) 2523–2525. [13] S. Kück, Laser-related spectroscopy of ion-doped crystals for tunable solidstate laser, Appl. Phys. B 72 (2001) 515–562. [14] T. Buch, B. Clerjaud, B. Lambert, P. Kovacs, Electron paramagnetic resonance study of 3d5 ions in mixed-polytype zinc sulfide, Phys. Rev. B 7 (1973) 184– 191. [15] P. Jaszczyn-Kopec, J. Gallagher, H. Kallmann, B. Kramer, Electron and hole trapping in ZnS phosphors investigated by electron-paramagnetic-resonance and luminescence methods, Phys. Rev. 140 (1965) A1309–A1316. [16] T. Estle, W. Holton, Electron-paramagnetic-resonance investigation of the superhyperfine structure of iron-group impurities in II–VI compounds, Phys. Rev. 150 (1966) 159–167. [17] J. Vallin, G. Watkins, EPR of Cr2+ in II–VI lattices, Phys. Rev. B 9 (1974) 2051– 2072. [18] M. Godlewski, M. Surma, V.Yu. Ivanov, T.P. Surkova, Mechanisms of radiative and nonradiative recombination in ZnSe:Cr and ZnSe:Fe, Low Temp. Phys. 30 (2004) 891–896. [19] M. Surma, A. Zakrzewski, M. Godlewski, Nonradiative recombination processes in nickel- and iron-doped ZnS and ZnSe studied by photoinduced electron-spin resonance, Phys. Rev. B 52 (1995) 11879–11883. [20] V. Ivanov, Y. Semenov, M. Surma, M. Godlewski, Anti-stokes luminescence in chromium-doped ZnSe, Phys. Rev. B 54 (1996) 4696–4701. [21] J.A. Weil, J.R. Bolton, J.E. Wertz, Electron Paramagnetic Resonance, John Wiley & Sons Inc, N.Y., 1994. [22] R.S. Title, Paramagnetic-resonance Spectra of the 3d5 Configuration of Chromium in ZnSe and ZnTe, Phys. Rev. 133 (1964) A1613–A1616. [23] M. Godlewski, On the application of the photo-EPR technique to the studies of photoionization, DAP recombination, and non-radiative recombination processes, Phys. Status Solidi A 90 (1985) 11–52. [24] J. Dashdorj, M.E. Zvanut, J.G. Harrison, Measurements of optical cross sections of the carbon vacancy in 4H-SiC by time-dependent photoelectron paramagnetic resonance, J. Appl. Phys. 104 (2008) 113707. [25] V.Yu. Ivanov, G. Karczewski, M. Godlewski, A.R. Omel’chuk, A.E. Belyaev, N.V. Zhavoronkov, Role of auger-type energy transfer processes in quenching of anti-stokes emission in chromium and iron doped ZnSe: ODMR, Optical and time-resolved study, Acta Phys. Pol. A 103 (2003) 695–701. [26] T. Förster, Zwischenmolekulare Energiewanderung und Fluoreszenz, Annalen der Physik 437 (2) (1948) 55–75. [27] D.L. Dexter, A theory of sensitized luminescence in solids, J. Chem. Phys. 21 (1953) 836–850. [28] D.L. Huber, Fluorescence in the presence of traps, Phys. Rev. B 20 (1979) 2307– 2314. [29] T. Miyakawa, D.L. Dexter, Phonon sidebands, multiphonon relaxation of excited states, and phonon-assisted energy transfer between ions in solids, Phys. Rev. B1 (1970) 2961–2969. [30] F. Auzel, Multiphonon processes, cross-relaxation, and upconversion in ionactivated solids, exemplified by minilaser materials, Radiationless Processes, in: B. Bartolo (Ed.), Plenum, New York, 1980, pp. 213–286. [31] G. Boulon, Spectroscopic Studies of Energy Transfer in Solids, in: Energy Transfer Processes in Condensed Matter NATO ASI Series v114, 1984, pp. 603– 620. [32] B. Henderson, R.H. Bartram, Crystal-Field Engineering of Solid-State Laser Materials, 1st ed., Cambridge University Press, UK, 2000. [33] B. Henderson, G.F. Imbusch, Optical Spectroscopy of Inorganic Solid, Clarendon, Oxford, 1989.