Jones

MATERIAL PREPARATION AND INFRARED SPECTROSCOPY OF Cr2+ DOPED II-VI SEMICONDUCTOR WINDOWS AND CRYSTALS FOR

MID-INFRARED LASER APPLICATIONS

Ivy Krystal Jonesa, Uwe Hommericha, S.B. Trivedib

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, Cd0.96Zn0.04Te, Cd0.90Zn0.10Te, Cd0.80Zn0.20Te, 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). Cr2+ 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 Cr2+ peak absorption coefficients ranging from ~0.8 cm-1 to 28.7 cm-1. The Cr2+ 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 Cr2+ concentrations above ~1x1019cm-3. By increasing the Zn content within the Cr: CdXZn1-XTe series, a significant shift of the Cr2+ absorption to shorter wavelengths was observed.

1

Jones

Introduction

In the last decade there has been an increased 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 (Cr2+, Co2+, Fe2+, etc…) doped zinc and cadmium chalcogenides (ZnS, ZnSe, ZnTe, CdSe, CdTe, etc… ) have been evaluated as an innovative class of laser media. 1-15 The results demonstrated that Cr2+ 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 operation, complexity, narrow wavelength coverage, and limited power scaling routes.1-3 In contrast to many other transition metal doped solids, Cr2+ ions in II-VI materials can exhibit significantly higher quantum yields at room temperature. Cr2+ ions are incorporated in a tetrahedral coordination in II-VI semiconductor 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 semiconductors directly effects the crystal-field energy level splitting, electron-phonon interactions, emission quantum yields, and provides radiative emission further into the MIR spectral region. Polycrystalline ZnSe and CdTe windows have proven to be promising candidates as host materials for Cr2+ ions. In addition, ternary Cd based II-VI materials are also being considered as novel Cr2+ laser hosts.13,14,15

Material Preparation

In addition to previous Cr diffusion doping experiments on ZnSe and CdTe windows9, Cr diffusion doping was also performed on a series of ternary CdZnTe compositions including Cd0.96Zn0.04Te, Cd0.90Zn0.10Te, and Cd0.80Zn0.20Te.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 Cr2+ 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, Cd0.96Zn0.04Te, Cd0.90Zn0.10Te, Cd0.80Zn0.20Te, and ZnTe were investigated. The Cr: CdZnTe series was evaluated for possible compositional effects, which may alter the spectroscopic properties of Cr2+ 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.

Figure 1: Absorption spectra of Cr2+:ZnSe and Cr2+:CdTe polycrystalline window materials. The samples are listed in table 1.

Results

The Cr2+ absorption spectra of the investigated samples were determined from the transmission spectra using Beer-Lambert Law.

(1)

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:Cd0.96Zn0.04Te, Cr:Cd0.90Zn0.10Te, Cr:Cd0.80Zn0.20Te, and Cr:ZnTe are depict in figure 2.

Figure 2: Absorption spectra of single crystals of Cr2+: CdTe, Cd0.96Zn0.04Te, Cd0.90Zn0.10Te, Cd0.80Zn0.20Te, and ZnTe.

The successful incorporation of Cr2+ ions into the II-VI windows is evidenced by the strong absorption bands centered at ~1750 nm for Cr2+:ZnSe and at ~1900 nm for Cr2+: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 Cr2+ 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 Cr2+ concentration in the samples was calculated by using equation (2):

(2)

where N is the Crconcentration and σ is the absorption cross-section. The absorption cross-sections for Cr: ZnSe has been reported to be 1.1 x 10-18 cm2 and the value for Cr: CdTe is 2.2x10-18cm2. 1-3

Figure 3: Comparison of emission spectra of Cr:ZnSe and Cr:CdTe windows.

As figure 3 shows, the emission from Cr2+:CdTe is shifted to a longer wavelength compared to Cr2+: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 ~3x1019cm-3 (see also table 1).

Figure 4 depicts the emission lifetime transients of Cr: ZnSe and Cr: CdTe.

Cr: ZnSe window / Absorption coefficient,
α [cm-1] / Cr2+ concentration
[cm-3] / Lifetime
τ
(µs)
1 / 28.7 / 2.6 x 1019 / 2.2
2 / 7.7 / 7.0 x 1018 / 5.7
3 / 5.6 / 5.1 x1018 / 6.5
4 / 3.7 / 3.4 x 1018 / 5.8
5 / 0.7 / 6.4 x 1017 / 5.3
Cr: CdTe
window / Absorption
coefficient,
α [cm-1] / Cr2+ concentration
[cm-3] / Lifetime
τ
(µs)
1 / 15.0 / 6.8 x 1018 cm-3 / 2.3
2 / 9.1 / 4.1 x 1018 cm-3 / 2.5
3 / 0.8 / 3.6 x 1017 cm-3 / 3.4
4 / 0.1 / 4.5 x 1016 cm-3 / ----

Table 1: Absorption coefficients, Cr2+ concentrations, and MIR emission lifetimes of Cr: ZnSe and Cr: CdTe windows.

Cr2+ doping experiments were also successfully performed on a series of single crystals of CdTe, Cd0.96Zn0.04Te, Cd0.90Zn0.10Te, Cd0.80Zn0.20Te and ZnTe using a thermal diffusion method. As shown in figure 3, it was observed that with increasing Zn content of CdZnTe, the Cr2+ 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.

Figure 5: MIR emission spectra of Cr2+: CdTe, Cd0.96Zn0.04Te, Cd0.90Zn0.10Te, Cd0.80Zn0.20Te, and ZnTe single crystals.

Conclusion

A diffusion method was applied for the preparation of Cr2+ doped polycrystalline ZnSe and CdTe windows as well as single crystals of Cr: CdTe, CdTe, Cd0.96Zn0.04Te, Cd0.90Zn0.10Te, Cd0.80Zn0.20Te and ZnTe. All samples exhibited the characteristic absorption and emission features of tetrahedrally coordinated Cr2+ 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.

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