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CdZnTe Impurity and Te-Precipitate Defect Reduction


OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Microelectronics The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws. OBJECTIVE: Improve the quality of CdZnTe substrates for HgCdTe growth by reducing the level of impurities in order to advance the production of the highest performing long-wave infrared (LWIR) detector arrays. DESCRIPTION: HgCdTe infrared detector array technology has improved significantly over the last decade. However, a current limitation of MBE HgCdTe epilayers is the presence of a surface micro-defect density in the 7x10^2- 1x10^4 cm^-2 range. These micro-defects have a diameter of about 2 micrometers in size and degrade detector device performance. Such MBE HgCdTe surface defects are induced by surface Te-precipitates present on the CdZnTe substrates. Commercially available (211) CdZnTe substrates for MBE HgCdTe have been grown specifically to have high Te-precipitate densities of about 5x10^6 – 1x10^7 cm^-3. The main reason for this is that such Te-precipitates are needed to getter and trap the very low levels of p-type acceptor impurities present on the bulk substrates and prevent them from diffusing and compensating/contaminating the MBE HgCdTe epilayers during device processing. It is hoped the need for substrate Te-precipitates can be reduced or fully eliminated during CdZnTe growth by purification improvements of the starting materials (Te, Cd, Zn, CdTe, ZnTe, and/or CdZnTe boules). Innovative ideas are requested for the reduction in p-type acceptor impurities like Cu, Li, Na, Au, etc. from the current state-of-the-art (SOA) of a few parts per billion (ppb) in the starting materials. Reductions of other common impurities from these starting materials are also encouraged. The proposed impurity analysis methodology should be described in detail. To detect the very low impurity levels required for 8N purity materials it is possible the standard chemical analysis technique of Glow Discharge Mass Spectrometry (GDMS) may not be sensitive enough to detect Cu and the other acceptor impurities mentioned above. If that is the case, then a direct or indirect alternate analytical approach should also be proposed. As a result of this effort, infrared Focal Plane Array (FPA) sensors with much higher yield would become available. Higher purity materials could also advance commercial CdTe-based solar cells by improving the device collection efficiency and production yields. Please note that this topic is focused on improving our capability to grow 8N (99.999999%) purity bulk single-crystal CdZnTe material. Solutions related to processing bulk-material into CdZnTe substrates (e.g., wire sawing, dicing, grinding, lapping, polishing) are outside the scope of this topic. Proposals should include a number of purification innovations that, as a whole, would significantly push the SOA. Proposed solutions should also be compatible with all the material specifications and safety requirements of a SOA commercial CdZnTe foundry. PHASE I: Study the scientific and technical feasibility of the proposed approach. Collaborate with government agencies and industry (e.g., starting material suppliers, CdZnTe foundries, and detector manufacturers) to develop requirements. Conduct research, analyses, and experimentation as needed to demonstrate feasibility and/or validate purification models. Develop preliminary designs for any new equipment, if applicable. Complete cost and performance assessments and compare to existing SOA approaches. Identify risk areas and mitigation plans that would be implemented in Phase II. Responders to this topic are strongly encouraged to team with existing starting material suppliers. Complete a plan for Phase II and contact starting material suppliers to verify the plan is executable. PHASE II: Finalize equipment purification designs and fabricate a prototype, if applicable. Demonstrate the ability to carry out further purification improvements of the starting materials (Te, Cd, Zn, CdTe, ZnTe, and/or CdZnTe boules) before they are used for high quality growth of single-crystal CdZnTe substrates meeting the topic objectives. Responders to this topic are strongly encouraged to team with existing starting material suppliers. Provide samples of the purified materials to the Government and industry partners for independent assessment. Update models with experimental data and refine the design based on lessons learned. Finalize cost and performance estimates based on these initial results. Collaborate with industry partners to put together a Phase III plan that includes quotes and letters of commitment. PHASE III DUAL USE APPLICATIONS: Transition operation of the purification and growth capability to CdZnTe commercial foundry operators. Provide supporting documentation and training for their operation and maintenance. Make multiple lots of single-crystal CdZnTe substrates for verification testing to demonstrate quality, consistency and reproducibility of the improved purity material. Show how the technology can also support CdZnTe growth for other defense and commercial applications (e.g. CdTe solar cells). REFERENCES: 1. J.M. Arias, M. Zandian, J. Bajaj, J.G. Pasko, L.O. Bubulac, S.H. Shin, and R.E. De Wames, J. Electron. Mater. 24, 521 (1995). 2. J. D. Benson, L. O. Bubulac, M. Jaime-Vasquez, J. M. Arias, P. J. Smith, R. N. Jacobs, J. K. Markunas, L. A. Almeida, A. Stoltz, P. S. Wijewarnasuriya, J. Peterson, M. Reddy, K. Jones, S. M. Johnson, and D. D. Lofgreen, J. Electron. Mater. 46, (2017). 3. Koyama, A., Hichiwa, A. & Hirano, R. Recent progress in CdZnTe crystals. J. Electron. Mater. 28, 683–687 (1999). 4. Benson, J.D., Bubulac, L.O., Smith, P.J. et al. Impact of Tellurium Precipitates in CdZnTe Substrates on MBE HgCdTe Deposition. J. Electron. Mater. 43, 3993–3998 (2014). 5. Vydyanath, H.R, Ellsworth, J.A., et al. (1993) J. Electron. Mater., 22, 1073. 6. Wijewarnasuriya, P.S., Zandian, M., Young, D.B., Arias, J.M., et al. Microscopic defects on MBE grown LWIR Hg1−xCdxTe material and their impact on device performance. J. Electron. Mater. 28, 649–653 (1999). 7. Korenstein, R., Olson, R.J., Lee, D. et al. (1995) J. J. Electron. Mater., 24, 511. KEYWORDS: CdZnTe; CZT; Material Purification; Purity Testing
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