# Presentation: 708944

## Crystal-Field Engineering of Transition Metal Doped Chalcogenide Laser MaterialsUwe Hömmerich, Hampton University, DMR-0301951

***Luminescent Materials Development for the 3-5 μm spectral region based on Co******2+****** doped binary and ternary II-VI materials.***

**The IR optical properties of Co****2+**** doped II-VI materials were investigated for applications in optoelectronics and lasers. Mid-IR emission bands in the 3-5 μm region were observed from several Co****2+**** doped binary and ternary II-VI semiconductors at room temperature. The 3-5 ********m range is of great interest for remote sensing of atmospheric constituents (e.g. CO****2****) and pollutants, bio-chemical detection, and IR countermeasures. The peak position of the mid-IR emission from Co****2+**** was compositional “tuned” through changes in the host material. The spectral changes reflect on the crystal-field strength experienced by Co****2+**** ions in II-VI hosts.**

**IR Absorption**

**Mid-IR Emission**

**Notes:**
“Crystal-Field Engineering of Transition Metal Doped Chalcogenide Laser Materials”, DMR-0301951, U. Hommerich/Hampton University Bulk crystals of transition metal (TM) doped II-VI materials continue to be of current interest for the development of novel mid-IR gain media for solid-state lasers. Compact mid-IR lasers operating beyond 3 μm are of technological interest for applications in IR countermeasures, bio-chemical detection, and atmospheric remote sensing. TM doped II-VI thin films are also currently being studied for the development of broadly tunable mid-IR lasers pumped through electrical carrier injection or impact excitation. Under support of this NSF award, the effects of compositional changes in the host material on the optical properties of divalent TM ions (e.g. Cr2+, Co2+, Fe2+) are being investigated. The overall goal of this work is to modify the optical properties of TM2+ ions for wavelength tailored applications.   The selection of dopants relates to the electronic configuration of the divalent transition metal ions (Cr2+: 3d4, Co2+: 3d7, Fe2+: 3d6) and the effect of the crystal field produced by the II-VI host. Based on earlier absorption studies on TM doped II-VI hosts, it is well established that for a given II-VI host, the crystal field splitting between the ground-state and first excited state decreases in the order of Cr2+, Co2+, and Fe2+.  The decrease in crystal-field splitting is expected to shift the emission from the first excited state to longer wavelengths in the series from Cr2+, Co2+, and Fe2+, as it was recently also reported for TM doped Zn-chalcogenides. During the time period of this report, we have extended the studies to Co2+ doped Cd-chalcogenides (CdTe and CdMnTe) and compared the obtained results to Co doped ZnSe. Moreover, the extension of Co2+ doping to a ternary II-VI material (e.g. CdMnTe) provides the opportunity to continously tune the crystal-field effect experienced by Co2+ ions, which can lead to changes in optical properties. 
Broad Mid-IR emission bands between 3-5μm were observed from all  Co2+ doped samples at room-temperature. As can be seen, the emission from Co: Cadmium Chalcogenides nicely overlaps with the absorption of Carbon-dioxide (CO2) at ~4.4μm, which is of interest for optical remote sensing.   The incorporation of Co2+ into Cadmium chalcogenides (CdTe, CdMnTe) was investigated using two doping methods, in-situ doping through melt growth or through thermal diffusion. Co2+ doping of CdTe/CdMnTe through melt growth was achieved, but a large portion of the resulting ingot suffered from high background absorption losses due to impurities and poor crystalline quality. As an alternative processing approach, undoped single crystals of CdTe and CdMnTe were grown using the Bridgman technique. The resulting ingots exhibited good quality single crystals with high transmission (~50-60%). The ingot was subsequently sliced into small pieces and Co2+ doping was performed through a post-growth thermal annealing process (thermal diffusion). Compared to melt growth, the annealing temperature for diffusion doping was ~200oC below the melting point of the material, which appears to have reduced the incorporation of unintended impurities. Co2+ was also successfully doped into polycrystalline windows of CdTe. The diffusion of Co2+ into single crystals and polycrystalline windows of Cd-chalcogenides provides an attractive and cost-effective method for obtaining good quality Co doped II-VI materials.Compared to Co: ZnSe, all Co doped Cadmium Chalcogenides exhibited red-shifted emission bands, consistent with predictions of crystal-field theory. For example, the mid-IR emission from Co: CdTe (crystals and windows) was shifted to longer wavelength by nearly 400 nm compared to Cr: ZnSe. The change in the peak position of the mid-IR emission is understood in terms of, or can be explained by, the change in the lattice parameters of the II-VI host. The interionic bond-length of ZnSe is ~2.5 Å compared to ~2.8 Å for Co: CdTe. As predicted by crystal-field theory, the longer bond length in CdTe compared to Zn-based chalcogenides leads to a weaker crystal field strength experienced by Co2+ ions and a corresponding “red” shift of the optical properties.  The small change in the peak emission between Co: CdTe and Co: Cd0.85Mn0.15Te indicates the possibility to fine tune the MIR emission properties for a wavelengths specific applications.  Spectroscopic studies on the room-temperature emission efficiency of Co2+ doped binary and ternary II-VI materials are still in progress and will continue during the course of this project. 

## Crystal-Field Engineering of Transition Metal Doped Chalcogenide Laser MaterialsUwe Hömmerich, Hampton University, DMR-0301951

_**EDUCATION & OUTREACH:**_
- Graduate Level:
_*Ivy Krystal Jones*_
*(Physics, M.S. program)*
_*Peter Amedzake*_
*(Physics, Ph.D. program)*
- Undergraduate Level:
_*Gigi Frey*_
*(Biology, sophomore)*
_*Natasha Ferguson*_* *
*(Chemistry, sophomore)*
_*Howard Brown*_
*(Computer Science, junior)*
*(Thomas Nelson Community College)*

_***Ivy K. Jones***_
- (physics grad. student, M.S.)
- Ivy continued her studies
- on the diffusion doping and optical
- spectroscopy of Cr doped II-VI
- materials. Ivy presented her research
- at the 2005 MRS spring meeting
- and 2005 APS meeting. In addition,
- Ivy gave a presentation at the 2005
- National Conference of Black
- Physics Students.

_***IR Laser Materials Development***_
_***Group at H.U.***_
- A program in education and research
- training in IR laser materials development
- is being offered at H.U. The training
- includes the material synthesis, optical
- spectroscopy, modeling, and laser
- performance testing. Recent project
- activities will be presented at the 2005
- Summer Meeting of the American
- Association of Physics Teachers.
- Outreach activities include mentoring
- in summer programs and science demos
- for students from local high schools.

- From left: EiEi Nyein, P. Amedzake.
- I.K. Jones, U. Hommerich