2+
Ivy Krystal Jones a , Uwe Hommerich a , S.B. Trivedi b a Department of Physics, Hampton University, Hampton, Virginia 23668 b Brimrose Corporation of America, Baltimore, Maryland 21236
Abstract
The material preparation and infrared spectroscopy of Cr diffusion doped zinc and cadmium chalcogenides including ZnSe, CdTe, Cd
0.96
Zn
0.04
Te, Cd
0.90
Zn
0.10
Te, Cd
0.80
Zn
0.20
Te, and ZnTe are reported. The materials were prepared by a thermal diffusion process controlled by temperature (750-850°C) and time (0.25-6 days). Cr 2+ doped II-VI semiconductors continue to be of significant interest as gain media in mid-infrared
(2-3 µm) solid-state lasers. Commercial CrSe powder of 99.5 % purity was used as the dopant source.
Various samples of polycrystalline Cr:ZnSe and Cr:CdTe windows were prepared with Cr 2+ peak absorption coefficients ranging from ~0.8 cm -1 to 28.7 cm -1 . The Cr 2+ room-temperature decay time varied between 5-6 µs for Cr:ZnSe and 2-3 μs for Cr:CdTe. Mid-infrared emission studies revealed the effect of dopant concentration quenching for Cr 2+ concentrations above ~1x10 19 cm -3 . By increasing the Zn content within the Cr: Cd
X
Zn
1-X
Te series, a significant shift of the Cr 2+ absorption to shorter wavelengths was observed.
Introduction
In the last decade there has been an increased hosts, which is in contrast to the octahedral coordination found in many transition metal doped oxide and fluoride crystals. The tetrahedral coordination of TM ions in II-VI interest in the development of new materials for a solid-state laser for mid-infrared (MIR) applications such as atmospheric remote-sensing, medical procedures, analytic spectroscopic techniques, and military technologies. Recently, transition metal ions (Cr 2+ , Co 2+ , Fe 2+ , etc…) doped zinc and cadmium chalcogenides (ZnS, semiconductors directly effects the crystal-field energy level splitting, electron-phonon interactions, emission quantum yields, and provides radiative emission further into the MIR
ZnSe, ZnTe, CdSe, CdTe, etc… ) have been evaluated as an innovative class of laser media.
1-15 The results demonstrated that Cr 2+ doped II-
VI materials exhibit high emission quantum efficiencies, high gain cross sections, laser operation at room temperature, and broad laser tunability. Present tunable MIR sources include
Tm:YLF, color center lasers, lead-salt diode lasers, gas and chemical lasers, and optical parametric oscillators which all are affected by inherent disadvantages such as cryogenic spectral region. Polycrystalline ZnSe and CdTe windows have proven to be promising candidates as host materials for Cr 2+ ions. In addition, ternary Cd based II-VI materials are also being considered as novel Cr 2+ laser hosts.
13,14,15
Material Preparation
In addition to previous Cr diffusion doping experiments on ZnSe and CdTe windows 9 , Cr diffusion doping was also performed on a series of ternary CdZnTe compositions including operation, coverage, and limited power scaling routes.
1-3 contrast to many other transition metal doped solids, Cr complexity, narrow wavelength
In
2+ ions in II-VI materials can exhibit significantly higher quantum yields at room temperature. Cr 2+ ions are incorporated in a tetrahedral coordination in II-VI semiconductor
Cd
0.96
Zn
0.04
Te, Cd
0.90
Zn
0.10
Te, and
Cd
0.80
Zn
0.20
Te.
15 Cr: ZnTe was included in this study for comparison to Cr: CdZnTe. Dopant source was CrSe with 99.5% purity. Samples were sealed under vacuum at ~10 -5 torr in quartz ampoules. The ampoules were placed in the center of a one zone horizontal furnace. The diffusion was controlled by the diffusion

temperature and time.
7-9 Diffusion temperatures were set between 750°C and 850°C and the diffusion time varied between 0.25-6 days.
Spectroscopic Measurements
Transmission and absorption measurements were performed using a Cary5000 Spectrophotometer.
A selective group of Cr doped ZnSe and CdTe windows possessing different absorption coefficients and Cr 2+ concentration were further evaluated for MIR laser applications. In addition, the absorption and emission properties of a series of Cr doped single crystals including CdTe,
Cd
0.96
Zn
0.04
Te, Cd
0.90
Zn
0.10
Te, Cd
0.80
Zn
0.20
Te, and
ZnTe were investigated. The Cr: CdZnTe series was evaluated for possible compositional effects, which may alter the spectroscopic properties of
Cr 2+ ions. The MIR emission measurements were performed using a Tm fiber laser operating at
1907 nm. The emission was detected with an
InSb detector and dispersed by a 0.3 m spectrometer with a 150 g/mm grating blazed at
2000 nm. Lifetime measurements were excited with the ~1675 nm output of an optical parametric oscillator pumped by a Nd:YAG laser. The emission was monitored broad band using spectral filters.
20
15
10
# 1
# 2
Cr:CdTe
5
0
# 3 # 4
40
Cr:ZnSe
30
# 5
# 3
20
# 2
10
# 4
# 1
0
1200 1400 1600 1800 2000 2200 2400 2600
Wavelength (nm)
Figure 1: Absorption spectra of Cr 2+ :ZnSe and
Cr 2+ :CdTe polycrystalline window materials.
The samples are listed in table 1.
Results
The Cr 2+ absorption spectra of the investigated samples were determined from the transmission spectra using Beer-Lambert Law.

(

)

T (

))
D where α is the absorption coefficient, T is the transmission as a function of wavelength, and D is the thickness of the sample. A series of samples with different Cr concentrations was prepared for Cr: ZnSe and Cr:CdTe windows.
9
The background corrected absorption spectra of
Cr:ZnSe and Cr:CdTe windows are shown in figure 1. The absorption data obtained from single crystals of Cr:CdTe, Cr:Cd
0.96
Zn
0.04
Te,
Cr:Cd
0.90
Zn
0.10
Te, Cr:Cd
0.80
Zn
0.20
Te,
Cr:ZnTe are depict in figure 2. and
4
Cr:CdTe 1910 nm
3
Cr:Cd
0.96
Zn
0.04
Te
1870 nm
2
Cr:Cd
0.90
Zn
0.10
Te
Cr:Cd
0.80
Zn
0.20
Te
Cr:ZnTe
1855 nm
1780 nm
1
0
1780 nm
1000 1500 2000 2500
Wavelength (nm)
Figure 2: Absorption spectra of single crystals of
Cr 2+ : CdTe, Cd
0.96
Zn
0.04
Te, Cd
0.90
Zn
0.10
Te,
Cd
0.80
Zn
0.20
Te, and ZnTe.
The successful incorporation of Cr 2+ ions into the
II-VI windows is evidenced by the strong absorption bands centered at ~1750 nm for
Cr 2+ :ZnSe and at ~1900 nm for Cr 2+ :CdTe. In most experiments it was observed that the absorption coefficient increased with increasing diffusion time and temperature. The shortest diffusion time of 6 hours at 750°C resulted in the smallest absorption coefficient of ~0.7cm
-1 . For example, diffusion conditions for Cr: CdTe

sample # 1 were six days at 850°C and sample
#3 was annealed for only three day at the same temperature. The decrease in diffusion time yielded a significant decrease in the peak absorption coefficient from ~15.0 cm -1 (sample
#1) to ~ 9.1 cm -1 (sample #3). The Cr 2+ absorption coefficient in Cr: ZnSe ranged from
~0.7 to ~28.7 cm -1 . For Cr: CdTe the absorption coefficients ranged from ~0.1 cm -1 to 15.0 cm -1
.
The Cr 2+ concentration in the samples was calculated by using equation (2):

N
 where N is the Cr concentration and σ is the absorption cross-section. The absorption crosssections for Cr: ZnSe has been reported to be
1.1 x 10
2.2x10
-
18
-18 cm 2 and the value for Cr: CdTe is cm 2 .
1-3
1.0
Cr
2+
:ZnSe
Cr
2+
:CdTe
0.8
0.6
0.4
0.2
0.0
750 o
C
850 o
C
2000 2200 2400 2600 2800 3000 3200 3400
Wavelength (nm)
Figure 3: Comparison of emission spectra of
Cr:ZnSe and Cr:CdTe windows.
As figure 3 shows, the emission from Cr 2+ :CdTe is shifted to a longer wavelength compared to
Cr 2+ :ZnSe by more then ~150 nm. In addition,
Cr: CdTe exhibited a broader emission spectrum compared to Cr: ZnSe. The emission decay transients of Cr: ZnSe and Cr: CdTe are depict in figure 4. For low Cr concentrations, the emission lifetime for Cr: ZnSe was ~5.3 µs and for Cr:
CdTe ~3 µs. For Cr: ZnSe the onset of Cr concentration quenching was apparent for concentrations larger then ~3x10 19 cm -3 (see also table 1).
1
0.1
#1
#2
#3
1
0.1
#1
#2 - #4
0.01
0 5
0.01
10 0 5 10 15
Time (  s)
Figure 4 depicts the emission lifetime transients of Cr: ZnSe and Cr: CdTe.
Cr:
ZnSe window
1
Absorption coefficient,
α [cm -1 ]
28.7
Cr 2+ concentration
[cm -3 ]
Lifetime
τ
(µs)
2.6 x 10 19 2.2
2
3
4
5
7.7
5.6
3.7
0.7
7.0 x 10 18
5.1 x10 18
3.4 x 10 18
6.4 x 10 17
5.7
6.5
5.8
5.3
Cr:
CdTe window
1
2
3
4
Absorption coefficient,
α [cm -1 ]
15.0
9.1
0.8
0.1
Cr 2+ concentration
[cm -3 ]
Lifetime
τ
(µs)
6.8 x 10 18 cm -3 2.3
4.1 x 10 18 cm -3 2.5
3.6 x 10 17 cm -3 3.4
4.5 x 10 16 cm -3 ----
Table 1: Absorption coefficients, Cr 2+ concentrations, and MIR emission lifetimes of
Cr: ZnSe and Cr: CdTe windows.
Cr 2+ doping experiments were also successfully performed on a series of single crystals of CdTe,
Cd
0.96
Zn
0.04
Te, Cd
0.90
Zn
0.10
Te, Cd
0.80
Zn
0.20
Te and
ZnTe using a thermal diffusion method. As

shown in figure 3, it was observed that with increasing Zn content of CdZnTe, the Cr 2+ absorption shifted to shorter wavelength. It can also be noticed that the Cr doped CdZnTe crystals have very similar emission features centered at ~ 2500nm (figure 5). Further spectroscopic studies on Cr: CdZnTe are still in progress.
5
Cr:CdTe
4
3
2
Cr:Cd
0.96
Zn
0.04
Te
Cr:Cd
0.90
Zn
0.10
Te
Cr:Cd
0.80
Zn
0.20
Te
1
Cr:ZnTe
0
2000 2200 2400 2600 2800 3000 3200 3400
Wavelength (nm)
Figure 5: MIR emission spectra of Cr 2+ : CdTe,
Cd
0.96
Zn
0.04
Te, Cd
0.90
Zn
0.10
Te, Cd
0.80
Zn
0.20
Te, and
ZnTe single crystals.
Conclusion
A diffusion method was applied for the preparation of Cr 2+ doped polycrystalline ZnSe and CdTe windows as well as single crystals of
Cr: CdTe, CdTe, Cd
0.96
Zn
0.04
Te, Cd
0.90
Zn
0.10
Te,
Cd
0.80
Zn
0.20
Te and ZnTe. All samples exhibited the characteristic absorption and emission features of tetrahedrally coordinated Cr 2+ ions.
The MIR emission properties of these materials are promising for solid-state laser development in the 2-3
 m spectral region. Further optimization of the materials preparation are needed to obtained laser quality samples for future MIR laser performance testing.
Acknowledgements
The authors from Hampton University acknowledge the support of the National Science
Foundation (DMR-0301951) and Virginia Space
Grant Consortium. I would especially like to thank Dr. Uwe Hömmerich, my advisor, for his constant encouragement and support in the development of this research. I am also extremely grateful to Dr. Sudhir Trivedi for his guidance and great ideas during the visit to
Brimrose Corporation of America.
References
1 L.D. DeLoach, R.H. Page, G.D. Wilke, S.A.
Payne, and W.F. Krupke, “Transition Metal-
Doped Zinc Chalcogenides: Spectroscopy and
Laser Demonstration of New Class of Gain
Media,” IEEE Journal Quantum Electronics , vol. 32, pp. 885-895, 1996.
2 I.T. Sorokina, “Cr 2+ -doped II-VI materials for lasers and nonlinear optics,” Optical Materials , vol. 26, pp. 395-412, 2004.
3 T.J. Carrig, “Transition-Metal-Doped
Chalcogenide Lasers,” Journal of. Electronic
Materials.
, vol. 31, pp. 759-769, 2002.
4 A.G. Bluiett, U. Hömmerich, R.T. Shah, S.B.
Trivedi, S.W. Kutcher, and C.C. Wang,
“Observation of Lasing from Cr 2+ : Doped II-VI
Semiconductors,” Journal of Electronic
Materials.
, vol. 31, pp. 806-810, 2002.
5 G. Grebe, G. Roussos, and H.-J. Schulz,
“Infrared luminescence of ZnSe:Cr crystals”, J.
Luminescence, Vol. 12/13, pp. 701 – 705, 1976.
6 V.I. Levchenko, V.N. Yakimovich, L.I
Postnova, V.I. Konstantinov, V.P. Mikhaillov, and N.V. Kuleshov, “Preparation and properties of bulk ZnSe:Cr single crystals,” Journal of
Crystal Growth , vol. 198/199, 980-983, 1999.
7 A. Burger, K. Chattopadhyay, J.-O. Ndap, X.
Ma, S.H. Morgan, C.I. Rablau, C.-H. Su, S. Feth,
R.H. Page, K.I. Schaffers, S.A. Payne,
“Preparation conditions of chromium doped
ZnSe and their infrared luminescence properties,” Journal of Crystal Growth , vol. 225, pp. 249 – 256, 2001.
8 J.-O. Ndap, K. Chattopadhyay, O.O. Adetunji,
D.E. Zelmon, A. Burger, “Thermal diffusion
Cr 2+ in bulk ZnSe,” Journal of Crystal Growth , vol. 240, pp. 176-184, 2002.

9 U. Hömmerich, I. K. Jones, E. E. Nyein, and S.
B. Trivedi, “Comparison of the optical properties of diffusion-doped polycrystalline Cr:ZnSe and
Cr:CdTe windows”, J. Crystal Growth, Vol. 287, pp. 450 – 453, 2006.
10 A.V. Podlipensky, V.G. Shcherbitsky, N.V.
Kuleshov, V.I. Levchenko, V.N. Yakimovich,
M. Mond, E. Heumann, G. Huber, H.
Kretschmann, S. Kück, “Efficient laser operation and continuous-wave diode pumping of
Cr 2+ :ZnSe single crystals,” Applied Physics B
Lasers and Optics , vol. 72, pp. 253-255, 2001.
11 E. Sorokin and I.T. Sorokina, “Tunable diodepumped continuous-wave Cr2+:ZnSe laser,”
Applied Physics Letters , vol. 80(18), pp. 3289-
3291, 2002.
12 A. Di Lieto, M. Tonelli, “Development of a cw polycrystalline Cr 2+ : ZnSe laser,” Optics and
Lasers in Engineering , vol. 39, pp. 305-308,
2003.
13 V. R. Davis, X. Wu, U. Hömmerich, K.
Grasza, S. B. Trivedi, and Z. Yu, “Optical properties of Cr 2+ ions in Cd
0.85
Mn
0.15
Te”, J.
Luminescence, Vol. 72 – 74, pp.281 – 283, 1997.
14 U. Hömmerich, J. T. Seo, A. Bluiett, M.
Turner, D. Temple, S. B. Trivedi, H. Zang, S. W.
Kutcher, C. C. Wang, R. J. Chen, and B.
Schumm, “Mid-infrared laser development based on transition metal doped cadmium manganese telluride”, J. Luminescence, Vol. 87 – 89, pp.
1143 – 1145, 2000.
15 U. Hömmerich, A. G. Bluiett, I. K. Jones, S.
B. Trivedi, and R. T. Shah, “Crystal growth and infrared spectroscopy of Cr:Cd
1-X
Zn
X
Te and
Cr:Cd
1-X
Mg
X
Te”, J. Crystal Growth, Vol. 287, pp. 243 – 247, 2006.