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CdTe synthesis and crystal growth using the high-pressure Bridgman technique
Journal Pre-proofs CdTe synthesis and crystal growth using the high-pressure Bridgman technique Tawfeeq K. Al-Hamdi, Seth W. McPherson, Santosh K. Swain, Joshah Jennings, Joel N. Duenow, X. Zheng, D.S. Albin, T. Ablekim, E. Colegrove, M. Amarasinghe, Andrew Ferguson, Wyatt K. Metzger, Csaba Szeles, Kelvin G. Lynn PII: DOI: Reference:
S0022-0248(19)30681-5 https://doi.org/10.1016/j.jcrysgro.2019.125466 CRYS 125466
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
18 July 2019 17 December 2019 27 December 2019
Please cite this article as: T.K. Al-Hamdi, S.W. McPherson, S.K. Swain, J. Jennings, J.N. Duenow, X. Zheng, D.S. Albin, T. Ablekim, E. Colegrove, M. Amarasinghe, A. Ferguson, W.K. Metzger, C. Szeles, K.G. Lynn, CdTe synthesis and crystal growth using the high-pressure Bridgman technique, Journal of Crystal Growth (2019), doi: https://doi.org/10.1016/j.jcrysgro.2019.125466
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
CdTe synthesis and crystal growth using the high-pressure Bridgman technique Tawfeeq K. Al-Hamdi1,2, Seth W. McPherson1, Santosh K. Swain1,a, Joshah Jennings1, Joel N. Duenow3, X. Zheng3, D.S. Albin3, T. Ablekim3, E. Colegrove3, M. Amarasinghe3, Andrew Ferguson3,Wyatt K. Metzger3, Csaba Szeles4, Kelvin G. Lynn1 1 2
Center for Materials Research, Washington State University, Pullman, WA 99163
Mustansiriya University, College of Engineering, Department of Materials, Baghdad, Iraq 3
National Renewable Energy Laboratory, Golden, CO 80401 4
Nious Technologies Inc., Pittsburgh, PA 15238
Abstract Efficient, safe and cost-effective synthesis of CdTe from elements is rather challenging in silica sealed ampoules due to the high vapor pressure of Cd. In this article, we report on the integrated synthesis and crystal growth of high-purity CdTe using the high pressure Bridgman (HPB) technique that is scalable to large volumes. The process lends itself for cost competitive industrial production of polycrystalline feedstock material for photovoltaics, sensors and electro-optic applications. Cadmium telluride (CdTe) crystals exceeding 1 kg in size were synthesized from elemental Cd and Te sources with purity comparable to state-of-the-art gamma ray detector crystals. In addition, synthesis of highly-doped CdTe feedstock for thin film photovoltaics applications demonstrating effective incorporation of group V (As, Sb) dopants was achieved at growth speeds of ~500 mm/hr. The technique may be applicable to produce other II-VI compounds with volatile components. Key Words: A1. Doping, A1. Solubility, A2. Growth from melt, A2. Single crystal growth, B1. Cadmium compounds, B2. Semiconducting materials, B3. Solar cells ----------------a
Corresponding author: Center for materials research, Dana Hall 102, Washington State University,
Pullman, WA 99164, USA, E-mail: [email protected], Phone: +1-509-335-7590
1
1. Introduction Cadmium telluride and its ternary and quaternary alloys such as CdZnTe, CdMnTe, CdSeTe, and CdZnSeTe, are wide band gap semiconductor materials used in a wide range of applications including room-temperature x-ray and gamma radiation detectors for defense, medical imaging, spectroscopy, electro-optic modulators, and photovoltaics [1-6]. Each of these applications have different requirements in terms of material specifications. X-ray and gammaray detector applications require relatively thick high-purity semi-insulating single crystals, whereas terrestrial photovoltaics requires undoped or doped feedstock of CdTe and CdSeTe powders for high-throughput vaporization for the fabrication of thin film polycrystalline PV films. For these different applications, it is necessary to tune the electrical resistivity over a range of nearly 10 orders of magnitude, from low resistivity p-type CdTe for solar cell devices to semiinsulating crystals for x-ray and gamma ray detectors. The applications also require high-purity materials to achieve high device fabrication yields and cost-competitive production. Synthesis of CdTe in large quantities from elemental Cd and Te is a well-known challenge and there are few successful reports in literature [7]. The formation of the high melting point CdTe compound from the low melting point elements tends to form pools of unreacted molten Cd and Te as the mixture is heated. Because of the high vapor pressure of elemental Cd, the pressure from the overheated pools of Cd can increase well above 1 atmosphere at high temperature. This poses explosion risks in traditional methods using sealed quartz ampoules [8]. The probability of forming unreacted Cd pools is higher in larger-volume loads so the scalability is limited by the strength of the quartz ampoules. CdTe synthesis methods and material quality naturally affect the cost of CdTe device technologies. Small-volume synthesis of CdTe in sealed silica ampoules tends to be expensive due to the high labor and consumable costs. It is important to explore alternative CdTe
2
synthesis processes to develop robust, scalable, low cost CdTe feedstock synthesis that provides high purity, doping control and the flexibility to grow from both Cd and Te rich melts when necessary. Group V (P, As, Sb) doping can enhance II-VI semiconductor applications, for example it is now considered a key strategy towards increasing p-type conduction and open-circuit voltage in thin-film photovoltaics implementing CdSeTe and CdTe. Incorporating dopants in the II-VI feedstock has shown promising results in terms of film doping [9, 10, 11]. However, due to the volatile nature of P and As, achieving high dopant incorporation is challenging in sealed silica ampoules, particularly when growing large volume feedstock. The high-pressure Bridgman (HPB) technique, which has been used in the past for CdZnTe crystal growth with boule size ~10 Kg [12], can retain these volatile dopants in the CdTe material due to the high inert gas pressure applied during synthesis. The use of a sturdy high-purity graphite crucible in a semi-sealed configuration and inert gas pressure well above the vapor pressure of the volatile elements Cd, P, and As suppresses the escape of these components from the crucible and ensures good stoichiometry control and dopant incorporation. The HPB process completely eliminates the possibility of explosion. Compared to the techniques that use sealed quartz ampoules, the HPB process allows large volume and safe production of high purity CdTe and CdZnTe compounds for the x-ray and gamma-ray detector industry, as well as heavily doped CdTe feedstock for the thin film solar industry. Additionally, because HPB is a melt growth technique it has the natural advantage of faster growth rates and reasonable single crystal yield. We demonstrate that the advantageous features of the HPB process can be exploited for the synthesis of large volume of CdTe from elemental Cd and Te sources, enabling a low-cost production of source material with controlled purity, stoichiometry, and dopant incorporation. 3
2. Experiment The CdTe synthesis experiments were performed in a vertical HPB furnace using 5N purity elemental Cd and Te shots as source material. The shot size varied between 1 to 5 mm. In the experiments where doping was performed, 5N purity cadmium arsenide and 4N cadmium antimonide were used as the dopant sources for As and Sb doping, respectively. The charge containing the dopants was well mixed before synthesis to ensure a homogeneous distribution. A conical shape, high purity graphite crucible of 4-inch inner diameter and 15-inch height was used for the synthesis after a vacuum bake out. In some of the synthesis experiments, the source materials were placed in a pyrolytic boron nitride crucible, which was inserted into the graphite crucible. The Te and Cd shots were loaded into the pBN crucible covered by a fitting lid, followed by mixing the shots inside the crucible by shaking several times. This was done in order to minimize localized clusters of Cd and thus minimize inhomogeneity in the reaction during the synthesis process. There were several advantages of using a smaller pBN crucible inside the outer graphite crucible. It was observed that after growth it was relatively easier to remove the ingot from the pBN crucible while the ingot was found to stick to the graphite crucible. Secondly, use of a smaller pBN crucible with a lid minimizes the open volume and therefore charge loss from the boule. Furthermore, pBN crucible may have a positive impact on the observed high purity of the crystal. The outer graphite crucible was covered with a lid which had a ~1 mm size horizontal opening to allow pressure equilibrium between the inner volume of the crucible and the high pressure chamber. The chamber was pressurized to 60 atm by high-purity Ar at room temperature. The source material mixture was heated up to 1160 ˚C at a rate of ~ 80-100 ˚C/hr. The chamber pressure gradually increased from 60 atm at room temperature to ~ 80 atm at the maximum furnace temperature of 1160 ˚C. Undoped and group-V doped boules up to 1.2 kg were synthesized using elemental Cd and Te, at various solidification rates in the 4 mm/hr-500 mm/hr range, with total 4
process time (synthesis, homogenization, solidification and cool down) varying between 16-68 hrs. The impurity concentrations in the produced CdTe boules were measured by glow discharge mass spectrometry (GDMS). 3. Results and Discussion Several undoped and group V doped CdTe boules were synthesized from the starting elements. Thermocouples placed near the crucible tip are used to monitor the temperature during the synthesis process. Fig. 1 shows the process temperature during the synthesis step (heat up). The exothermic formation of CdTe is observed to start in the range of (420-450) C, which is near the melting point of Te. The measured weight loss (weight of the boule after synthesis and growth in growth experiments compared to the starting source material weight) in all the boules was < 1%, which was similar to what is typically obtained in our sealed quartz-based growths. However, in an ingot where the highest excess Cd (~53.45 % atomic) was used, the weight loss was 3.7%.
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Figure 1. (a) Exothermic melting of Te observed during HPB synthesis. The curves marked as Tip and Shoulder represent the corresponding thermocouple locations relative to the crucible. (b) The heat released during the formation process coincides with reduction in the power supplied to heaters indicating exothermic synthesis.
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The fastest (~500 mm/hr) solidified boule was characterized by powder X-ray diffraction (XRD) as shown in Fig. 2. The starting material consisted of elemental Cd and Te with excess Cd such that the overall charge contained 53.45% atomic Cd. Despite the high off-stoichiometry in the melt composition and the rapid solidification rate of 500 mm/hr, the material exhibits single phase CdTe with high structural perfection as observed by sharp diffraction peaks. Secondary phases, if any, may be present in the crystal below 0.3 wt%.
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HPB CdTe bulk crystals Growth rate: 500 mm/hr (220)
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Figure 2, X-ray diffraction pattern of samples from HPB CdTe grown at ~500 mm/hr with 53.45% atomic Cd in the initial composition. Some CdTe boules were grown at relatively slow rates of ~ 4mm/hr to evaluate the single crystalline volume that can be achieved by our HPB system. Crystals with diameters of ~ 60 – 100 mm and lengths up to ~90 mm grown from the starting elements is shown in Fig. 3a. Large grains extending over half of the ingot length are observed. For comparison, another ingot grown in a sealed quartz ampoule with the vertical Bridgman (VB) technique, at a rate of ~2mm/hr., is shown in Fig. 3b. It is usually difficult to achieve such large single crystal volume by known bulk growth techniques, including those from melt and solution, without seeding. The poor single crystal yield 6
is attributed to several factors such as poor thermal conductivity of solid CdTe, which causes the solid-liquid interface shape to be concave towards the melt. This in turn allows for inward grain growth from the crucible wall, crucible wall nucleation, and growth interface instability caused by constitutional undercooling. The latter is more pronounced in growth methods such as the travelling heater method where crystallization occurs from highly off-stoichiometric solution. Appropriate adjustments to growth parameters are usually made, such as slowing down the growth rate and imposing large temperature gradients to circumvent growth instability issues. This work demonstrates that large single crystal volumes, comparable to the typically observed ingot grain structures in other known bulk growth techniques, can be obtained by HPB process at faster rates, while synthesizing from elements in the same process step with all reusable parts. Suitably treated pyrolytic boron nitride crucibles were employed to minimize the melt adhesion to the crucible wall and minimize wall nucleation. In addition, a large temperature gradient and use of a graphite crucible support system may have promoted axial heat draw, resulting in favorable interface shape and thus large grains.
(a)
(b)
Figure 3, Comparison of (a) 4mm/hr melt grown CdTe:Sb ingot from a vertical high pressure Bridgman system after synthesis from elemental Cd and Te in the HPB system, and (b) 2mm/hr CdTe:Sb ingot grown in a sealed quartz ampoule in a Vertical Bridgman system using polycrystalline previously synthesized polycrystalline CdTe material. 7
While the actual single crystal volume is difficult to estimate as it will depend on the bulk grain structure. However, comparison with grain structure of crystal grown from melt in sealed quartz based method at similar growth rates, the HPB crystal have comparable surface grain sizes. The grains are several square centimeters in area and thus HPB can be suitable technique for II-VI alloy growths for single crystal research in addition to producing polycrystalline feedstock for thin film PV applications. The purity of the crystals was characterized by GDMS. Impurities can create compensating defects and adversely impact the intended electrical properties. Minimizing the concentration of impurities in the synthesized CdTe boules especially those producing deep levels in the band gap is critical for the radiation detector and PV applications. The problem is severe in material systems such as CdTe with a high melting points, which can promote impurity diffusion from the crucible. Solution based methods are known to achieve higher CdTe purity due to lower growth temperature and the strong purification effect of the solvent such as Te. The disadvantage of solution growth and synthesis processes such as the travelling heater method is the order of magnitude lower growth rates (3-4 mm/day). We found that the HPB technique achieves purity comparable to those required for gamma-ray detector grade materials, at growth rates of 4mm/hr or higher. Performing the integrated process of crystal growth after synthesis from elements in a single crucible and system eliminates contamination associated with handling and longer exposure times to containers when separating the synthesis and growth processes. Fig. 4(a) shows the GDMS purity analysis results on two HPB-grown crystals (CdTe:Sb HPB1, CdTe:Sb HPB2) compared with CdTe crystals grown in sealed quartz ampoule in a vertical Bridgman (VB) technique. In the latter, 99.99995% purity THM grown polycrystalline CdTe ingot chunks (5N Plus Inc.) were used as the starting material. The VB crystals include a phosphorus-doped crystal (CdTe:P VB) which 8
produced a record open-circuit voltage exceeding 1 Volt in single crystal solar cells[13], and a gamma ray detector grade crystal (CdTe:In VB) grown under excess Te, with excellent electron mobility-lifetime (µτ) >1x10-2 cm2/V, which is comparable to state-of-the-art commercial gammaray detectors. VB grown CdTe:As and CdTe:Sb crystals are also presented for comparison. The analysis indicates sufficiently high overall purity (<1 ppm total impurity concentration) suitable for high performance devices is achievable using HPB in a scalable and fast growth method that is capable of elemental synthesis and growth in a single integrated process. The purity results in this study are comparable or better than those reported in low temperature solution grown and vertical gradient freeze (VGF) grown CdTe [14, 15]. Fig. 4(b) indicates individual impurity concentrations measured by GDMS. Most of the known CdTe shallow acceptor and donor impurities (Na, Al, P, Cl etc) are detected in concentrations more than two orders of magnitude lower than the intended dopant and therefore are not clearly detrimental. On the other hand, some of the metallic impurities, Fe, Co, Ni, and Cu, which can induce deep level defects in CdTe and related II-VI compounds [16-19] are observed both in the sealed quartz grown and in the HPB grown materials, with higher concentrations of these impurities detected in heavily doped materials. This suggests that the lower purity dopants could be a potential contamination sources for these impurities and higher purity could be achieved using high purity dopants and Cd, Te elements.
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Figure 4, (a) Comparison of total impurity concentration between HPB grown CdTe:Sb crystals and various doped CdTe grown in sealed quartz ampoules in VB technique. Purity of crystals which has produced high performing single crystal solar cell and gamma ray detectors are also shown. (b) Impurity concentrations in the two HPB-grown and a VB-grown CdTe:Sb crystals. Both undoped and doped HPB grown feedstock have been used to produce solar cells by CdTe vapor transport deposition (VTD) [9] over a wide range of conditions with efficiencies ranging from 13–18%. The deposition was made on a glass substrate coated with transparent conductive oxide (TCO). The CdSeTe:As layer was deposited by close space sublimation whereas the CdTe;As layer was deposited by vapor transport deposition (VTD) technique. The performance using HPB grown feedstock is comparable to that produced by the Bridgman method. However, HPB provides more flexibility in modifying dopant levels to optimize performance [20] as well as a path to larger scale and lower costs.
For example, HPB boules were synthesized from the
elements with As and then used as source material to grow thin CdTe films by VTD. Secondary ion mass spectrometry (SIMS) indicates As incorporation levels of 1016–1018 cm-3 can be achieved (e.g. Fig 5) in both CdSeTe and CdTe. By varying the As levels in the feedstock or deposition
10
conditions, As incorporation levels can be increased or decreased, and hole density as high as 1017 cm-3 can be achieved. At the same time, minority carrier lifetime up to 40 ns is obtained from the device shown in figure 5(d), which is commensurate with state of the art CdTe thin film devices. The methods can be applied similarly for different dopants such as Sb and In, and useful for other applications.
(a)
(b)
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Figure 5, (a) Device structure of the thin film CdTe/CdSeTe doped with As. (b) SIMS depth profile of As incorporation. Both CdTe:As and CdSeTe:As source material were prepared by HPB. (c) Hole density obtained by C-V measurement and (d) Life-time measured by 2 photon excitation.
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4. Conclusion The results indicate that the HPB technique is an efficient melt-growth process that can maintain good control of purity and stoichiometry. CdTe with grain size comparable to ingots formed in sealed quartz ampoules by the MVB technique were synthesized at fast growth rates by HPB and coupled with Se alloying and effective II-VI semiconductor doping. The HPB capability to process tens of kilograms of material in a single batch offers avenues for enhancing material properties, scaling to manufacturing, and reducing costs for electro-optical applications. Acknowledgement The authors would like to thank Muad Saleh for performing the XRD measurements. The authors acknowledge financial support from U.S. Department of energy, Office of Energy Efficiency and Renewable Energy under Contracts DE-EE0007537 and DE-AC36-08GO28308. Tawfeeq K. Al-Hamdi would like to thank the Iraqi Ministry of Higher Education and Scientific Research and Mustansiriya University-College of Engineering for sponsoring his Ph.D. study in Washington State University (WSU)- School of Mechanical and Materials Engineering.
References 1. 2. 3. 4. 5. 6.
Takahashi, T. and S. Watanabe, Recent progress in CdTe and CdZnTe detectors. IEEE Transactions on Nuclear Science, 2001. 48(4): p. 950-959. Roy, U.N., et al., Growth and characterization of detector-grade CdMnTe by the vertical Bridgman technique. AIP Advances, 2018. 8(10): p. 105012. Roy, U.N., et al., Growth and characterization of Cd1-xZnxSeyTe1-y for radiation detector applications (Conference Presentation). SPIE Optical Engineering + Applications. Vol. 9968. 2016: SPIE. 1. Munshi, A.H., et al., Polycrystalline CdSeTe/CdTe Absorber Cells With 28 mA/cm2 Short-Circuit Current. IEEE Journal of Photovoltaics, 2018. 8(1): p. 310-314. Wu, X., High-efficiency polycrystalline CdTe thin-film solar cells. Solar Energy, 2004. 77(6): p. 803814. Iniewski, K., CZT detector technology for medical imaging. Journal of Instrumentation, 2014. 9(11): p. C11001-C11001. 12
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Zappettini, A. et. al, A new process for synthesizing high-purity stoichiometric cadmium telluride, Journal of Crystal Growth 214/215 (2000), pp. 14-18. Triboulet, R., Fundamentals of the CdTe synthesis. Journal of Alloys and Compounds, 2004. 371(1): p. 67-71. McCandless, B.E., et al., Overcoming Carrier Concentration Limits in Polycrystalline CdTe Thin Films with In Situ Doping. Scientific Reports, 2018. 8(1): p. 14519. Danielson, A., et al. Doping CdTe Absorber Cells using Group V Elements. in 2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC). 2018. Metzger, W., et al., Exceeding 20% efficiency with in-situ group V doping in polycrystalline CdTe solar cells, Nature Energy, 2019, 4, p. 837. Szeles, C. and E.E. Eissler, Advances in the crystal growth and device fabrication technology of CdZnTe room temperature radiation detectors. IEEE Transactions on Nuclear Science, Vol. 51, No. 3, 2004, pp. 1242-1249. Burst, J.M., et al., CdTe solar cells with open-circuit voltage breaking the 1 V barrier. Nature Energy, 2016. 1: p. 16015. A. Nagaoka, Kyu-Bum Han, S. Misra, T. Wilenski, T. Sparks and M. Scarpulla, Growth and characterization of Arsenic doped CdTe single crystals grown by Cd-solvent traveling-heater method, Journal of Crystal Growth 467 (2017) 6-11. C. Su, S. L. Lehoczky, B. Raghothamachar, and M. Dudley, Crystal growth and characterization of CdTe grown by vertical gradient freeze, Materials Science and Engineering B 147 (2008) 35–42. Kaufmann, U., J. Windscheif, and G. Brunthaler, Identification of the isolated deep Ni acceptor in CdTe and ZnTe: comparison with isomorphous systems. Journal of Physics C: Solid State Physics, 1984. 17(34): p. 6169-6176. Rzepka, E., et al., Deep centres for optical processing in CdTe. Materials Science and Engineering: B, 1993. 16(1): p. 262-267. Balcioglu, A., R.K. Ahrenkiel, and F. Hasoon, Deep-level impurities in CdTe/CdS thin-film solar cells. Journal of Applied Physics, 2000. 88(12): p. 7175-7178. Jr., R.C.B. and D.E. Cooper, Detection of dilute iron impurities in CdTe. Applied Physics Letters, 1988. 53(16): p. 1521-1523. A. Kanevce, M.O.R., T. M. Barnes, S. A. Jensen, and W. K. Metzger, The roles of carrier concentration and interface, bulk, and grain-boundary recombination for 25% efficient CdTe solar cells. Journal of Applied Physics, 2017. 121: p. 214506.
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Highlights 1. High pressure Bridgman technique is suitable to synthesize CdTe from elemental sources 2. Highly volatile dopants can be efficiently incorporated into melt grown CdTe under high pressure 3. High purity level in CdTe is achievable in scalable HPB melt growth process
Author contributions S. Swain, T. Al-Hamdi, S. Mcpherson, J. Jennings, C. Szeles, K. Lynn- Performed crystal growth in high pressure Bridgman furnace J. Duenow, X-Zheng, D. S. Albin, T. Ablekim, M. Amarasinghe, A. Ferguson, W. Metzger- Performed the thin film deposition, cell fabrication and measurements S. Swain, W. K. Metzger- Wrote the manuscript
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S0022-0248(19)30681-5 https://doi.org/10.1016/j.jcrysgro.2019.125466 CRYS 125466
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
18 July 2019 17 December 2019 27 December 2019
Please cite this article as: T.K. Al-Hamdi, S.W. McPherson, S.K. Swain, J. Jennings, J.N. Duenow, X. Zheng, D.S. Albin, T. Ablekim, E. Colegrove, M. Amarasinghe, A. Ferguson, W.K. Metzger, C. Szeles, K.G. Lynn, CdTe synthesis and crystal growth using the high-pressure Bridgman technique, Journal of Crystal Growth (2019), doi: https://doi.org/10.1016/j.jcrysgro.2019.125466
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
CdTe synthesis and crystal growth using the high-pressure Bridgman technique Tawfeeq K. Al-Hamdi1,2, Seth W. McPherson1, Santosh K. Swain1,a, Joshah Jennings1, Joel N. Duenow3, X. Zheng3, D.S. Albin3, T. Ablekim3, E. Colegrove3, M. Amarasinghe3, Andrew Ferguson3,Wyatt K. Metzger3, Csaba Szeles4, Kelvin G. Lynn1 1 2
Center for Materials Research, Washington State University, Pullman, WA 99163
Mustansiriya University, College of Engineering, Department of Materials, Baghdad, Iraq 3
National Renewable Energy Laboratory, Golden, CO 80401 4
Nious Technologies Inc., Pittsburgh, PA 15238
Abstract Efficient, safe and cost-effective synthesis of CdTe from elements is rather challenging in silica sealed ampoules due to the high vapor pressure of Cd. In this article, we report on the integrated synthesis and crystal growth of high-purity CdTe using the high pressure Bridgman (HPB) technique that is scalable to large volumes. The process lends itself for cost competitive industrial production of polycrystalline feedstock material for photovoltaics, sensors and electro-optic applications. Cadmium telluride (CdTe) crystals exceeding 1 kg in size were synthesized from elemental Cd and Te sources with purity comparable to state-of-the-art gamma ray detector crystals. In addition, synthesis of highly-doped CdTe feedstock for thin film photovoltaics applications demonstrating effective incorporation of group V (As, Sb) dopants was achieved at growth speeds of ~500 mm/hr. The technique may be applicable to produce other II-VI compounds with volatile components. Key Words: A1. Doping, A1. Solubility, A2. Growth from melt, A2. Single crystal growth, B1. Cadmium compounds, B2. Semiconducting materials, B3. Solar cells ----------------a
Corresponding author: Center for materials research, Dana Hall 102, Washington State University,
Pullman, WA 99164, USA, E-mail: [email protected], Phone: +1-509-335-7590
1
1. Introduction Cadmium telluride and its ternary and quaternary alloys such as CdZnTe, CdMnTe, CdSeTe, and CdZnSeTe, are wide band gap semiconductor materials used in a wide range of applications including room-temperature x-ray and gamma radiation detectors for defense, medical imaging, spectroscopy, electro-optic modulators, and photovoltaics [1-6]. Each of these applications have different requirements in terms of material specifications. X-ray and gammaray detector applications require relatively thick high-purity semi-insulating single crystals, whereas terrestrial photovoltaics requires undoped or doped feedstock of CdTe and CdSeTe powders for high-throughput vaporization for the fabrication of thin film polycrystalline PV films. For these different applications, it is necessary to tune the electrical resistivity over a range of nearly 10 orders of magnitude, from low resistivity p-type CdTe for solar cell devices to semiinsulating crystals for x-ray and gamma ray detectors. The applications also require high-purity materials to achieve high device fabrication yields and cost-competitive production. Synthesis of CdTe in large quantities from elemental Cd and Te is a well-known challenge and there are few successful reports in literature [7]. The formation of the high melting point CdTe compound from the low melting point elements tends to form pools of unreacted molten Cd and Te as the mixture is heated. Because of the high vapor pressure of elemental Cd, the pressure from the overheated pools of Cd can increase well above 1 atmosphere at high temperature. This poses explosion risks in traditional methods using sealed quartz ampoules [8]. The probability of forming unreacted Cd pools is higher in larger-volume loads so the scalability is limited by the strength of the quartz ampoules. CdTe synthesis methods and material quality naturally affect the cost of CdTe device technologies. Small-volume synthesis of CdTe in sealed silica ampoules tends to be expensive due to the high labor and consumable costs. It is important to explore alternative CdTe
2
synthesis processes to develop robust, scalable, low cost CdTe feedstock synthesis that provides high purity, doping control and the flexibility to grow from both Cd and Te rich melts when necessary. Group V (P, As, Sb) doping can enhance II-VI semiconductor applications, for example it is now considered a key strategy towards increasing p-type conduction and open-circuit voltage in thin-film photovoltaics implementing CdSeTe and CdTe. Incorporating dopants in the II-VI feedstock has shown promising results in terms of film doping [9, 10, 11]. However, due to the volatile nature of P and As, achieving high dopant incorporation is challenging in sealed silica ampoules, particularly when growing large volume feedstock. The high-pressure Bridgman (HPB) technique, which has been used in the past for CdZnTe crystal growth with boule size ~10 Kg [12], can retain these volatile dopants in the CdTe material due to the high inert gas pressure applied during synthesis. The use of a sturdy high-purity graphite crucible in a semi-sealed configuration and inert gas pressure well above the vapor pressure of the volatile elements Cd, P, and As suppresses the escape of these components from the crucible and ensures good stoichiometry control and dopant incorporation. The HPB process completely eliminates the possibility of explosion. Compared to the techniques that use sealed quartz ampoules, the HPB process allows large volume and safe production of high purity CdTe and CdZnTe compounds for the x-ray and gamma-ray detector industry, as well as heavily doped CdTe feedstock for the thin film solar industry. Additionally, because HPB is a melt growth technique it has the natural advantage of faster growth rates and reasonable single crystal yield. We demonstrate that the advantageous features of the HPB process can be exploited for the synthesis of large volume of CdTe from elemental Cd and Te sources, enabling a low-cost production of source material with controlled purity, stoichiometry, and dopant incorporation. 3
2. Experiment The CdTe synthesis experiments were performed in a vertical HPB furnace using 5N purity elemental Cd and Te shots as source material. The shot size varied between 1 to 5 mm. In the experiments where doping was performed, 5N purity cadmium arsenide and 4N cadmium antimonide were used as the dopant sources for As and Sb doping, respectively. The charge containing the dopants was well mixed before synthesis to ensure a homogeneous distribution. A conical shape, high purity graphite crucible of 4-inch inner diameter and 15-inch height was used for the synthesis after a vacuum bake out. In some of the synthesis experiments, the source materials were placed in a pyrolytic boron nitride crucible, which was inserted into the graphite crucible. The Te and Cd shots were loaded into the pBN crucible covered by a fitting lid, followed by mixing the shots inside the crucible by shaking several times. This was done in order to minimize localized clusters of Cd and thus minimize inhomogeneity in the reaction during the synthesis process. There were several advantages of using a smaller pBN crucible inside the outer graphite crucible. It was observed that after growth it was relatively easier to remove the ingot from the pBN crucible while the ingot was found to stick to the graphite crucible. Secondly, use of a smaller pBN crucible with a lid minimizes the open volume and therefore charge loss from the boule. Furthermore, pBN crucible may have a positive impact on the observed high purity of the crystal. The outer graphite crucible was covered with a lid which had a ~1 mm size horizontal opening to allow pressure equilibrium between the inner volume of the crucible and the high pressure chamber. The chamber was pressurized to 60 atm by high-purity Ar at room temperature. The source material mixture was heated up to 1160 ˚C at a rate of ~ 80-100 ˚C/hr. The chamber pressure gradually increased from 60 atm at room temperature to ~ 80 atm at the maximum furnace temperature of 1160 ˚C. Undoped and group-V doped boules up to 1.2 kg were synthesized using elemental Cd and Te, at various solidification rates in the 4 mm/hr-500 mm/hr range, with total 4
process time (synthesis, homogenization, solidification and cool down) varying between 16-68 hrs. The impurity concentrations in the produced CdTe boules were measured by glow discharge mass spectrometry (GDMS). 3. Results and Discussion Several undoped and group V doped CdTe boules were synthesized from the starting elements. Thermocouples placed near the crucible tip are used to monitor the temperature during the synthesis process. Fig. 1 shows the process temperature during the synthesis step (heat up). The exothermic formation of CdTe is observed to start in the range of (420-450) C, which is near the melting point of Te. The measured weight loss (weight of the boule after synthesis and growth in growth experiments compared to the starting source material weight) in all the boules was < 1%, which was similar to what is typically obtained in our sealed quartz-based growths. However, in an ingot where the highest excess Cd (~53.45 % atomic) was used, the weight loss was 3.7%.
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Figure 1. (a) Exothermic melting of Te observed during HPB synthesis. The curves marked as Tip and Shoulder represent the corresponding thermocouple locations relative to the crucible. (b) The heat released during the formation process coincides with reduction in the power supplied to heaters indicating exothermic synthesis.
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The fastest (~500 mm/hr) solidified boule was characterized by powder X-ray diffraction (XRD) as shown in Fig. 2. The starting material consisted of elemental Cd and Te with excess Cd such that the overall charge contained 53.45% atomic Cd. Despite the high off-stoichiometry in the melt composition and the rapid solidification rate of 500 mm/hr, the material exhibits single phase CdTe with high structural perfection as observed by sharp diffraction peaks. Secondary phases, if any, may be present in the crystal below 0.3 wt%.
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HPB CdTe bulk crystals Growth rate: 500 mm/hr (220)
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Figure 2, X-ray diffraction pattern of samples from HPB CdTe grown at ~500 mm/hr with 53.45% atomic Cd in the initial composition. Some CdTe boules were grown at relatively slow rates of ~ 4mm/hr to evaluate the single crystalline volume that can be achieved by our HPB system. Crystals with diameters of ~ 60 – 100 mm and lengths up to ~90 mm grown from the starting elements is shown in Fig. 3a. Large grains extending over half of the ingot length are observed. For comparison, another ingot grown in a sealed quartz ampoule with the vertical Bridgman (VB) technique, at a rate of ~2mm/hr., is shown in Fig. 3b. It is usually difficult to achieve such large single crystal volume by known bulk growth techniques, including those from melt and solution, without seeding. The poor single crystal yield 6
is attributed to several factors such as poor thermal conductivity of solid CdTe, which causes the solid-liquid interface shape to be concave towards the melt. This in turn allows for inward grain growth from the crucible wall, crucible wall nucleation, and growth interface instability caused by constitutional undercooling. The latter is more pronounced in growth methods such as the travelling heater method where crystallization occurs from highly off-stoichiometric solution. Appropriate adjustments to growth parameters are usually made, such as slowing down the growth rate and imposing large temperature gradients to circumvent growth instability issues. This work demonstrates that large single crystal volumes, comparable to the typically observed ingot grain structures in other known bulk growth techniques, can be obtained by HPB process at faster rates, while synthesizing from elements in the same process step with all reusable parts. Suitably treated pyrolytic boron nitride crucibles were employed to minimize the melt adhesion to the crucible wall and minimize wall nucleation. In addition, a large temperature gradient and use of a graphite crucible support system may have promoted axial heat draw, resulting in favorable interface shape and thus large grains.
(a)
(b)
Figure 3, Comparison of (a) 4mm/hr melt grown CdTe:Sb ingot from a vertical high pressure Bridgman system after synthesis from elemental Cd and Te in the HPB system, and (b) 2mm/hr CdTe:Sb ingot grown in a sealed quartz ampoule in a Vertical Bridgman system using polycrystalline previously synthesized polycrystalline CdTe material. 7
While the actual single crystal volume is difficult to estimate as it will depend on the bulk grain structure. However, comparison with grain structure of crystal grown from melt in sealed quartz based method at similar growth rates, the HPB crystal have comparable surface grain sizes. The grains are several square centimeters in area and thus HPB can be suitable technique for II-VI alloy growths for single crystal research in addition to producing polycrystalline feedstock for thin film PV applications. The purity of the crystals was characterized by GDMS. Impurities can create compensating defects and adversely impact the intended electrical properties. Minimizing the concentration of impurities in the synthesized CdTe boules especially those producing deep levels in the band gap is critical for the radiation detector and PV applications. The problem is severe in material systems such as CdTe with a high melting points, which can promote impurity diffusion from the crucible. Solution based methods are known to achieve higher CdTe purity due to lower growth temperature and the strong purification effect of the solvent such as Te. The disadvantage of solution growth and synthesis processes such as the travelling heater method is the order of magnitude lower growth rates (3-4 mm/day). We found that the HPB technique achieves purity comparable to those required for gamma-ray detector grade materials, at growth rates of 4mm/hr or higher. Performing the integrated process of crystal growth after synthesis from elements in a single crucible and system eliminates contamination associated with handling and longer exposure times to containers when separating the synthesis and growth processes. Fig. 4(a) shows the GDMS purity analysis results on two HPB-grown crystals (CdTe:Sb HPB1, CdTe:Sb HPB2) compared with CdTe crystals grown in sealed quartz ampoule in a vertical Bridgman (VB) technique. In the latter, 99.99995% purity THM grown polycrystalline CdTe ingot chunks (5N Plus Inc.) were used as the starting material. The VB crystals include a phosphorus-doped crystal (CdTe:P VB) which 8
produced a record open-circuit voltage exceeding 1 Volt in single crystal solar cells[13], and a gamma ray detector grade crystal (CdTe:In VB) grown under excess Te, with excellent electron mobility-lifetime (µτ) >1x10-2 cm2/V, which is comparable to state-of-the-art commercial gammaray detectors. VB grown CdTe:As and CdTe:Sb crystals are also presented for comparison. The analysis indicates sufficiently high overall purity (<1 ppm total impurity concentration) suitable for high performance devices is achievable using HPB in a scalable and fast growth method that is capable of elemental synthesis and growth in a single integrated process. The purity results in this study are comparable or better than those reported in low temperature solution grown and vertical gradient freeze (VGF) grown CdTe [14, 15]. Fig. 4(b) indicates individual impurity concentrations measured by GDMS. Most of the known CdTe shallow acceptor and donor impurities (Na, Al, P, Cl etc) are detected in concentrations more than two orders of magnitude lower than the intended dopant and therefore are not clearly detrimental. On the other hand, some of the metallic impurities, Fe, Co, Ni, and Cu, which can induce deep level defects in CdTe and related II-VI compounds [16-19] are observed both in the sealed quartz grown and in the HPB grown materials, with higher concentrations of these impurities detected in heavily doped materials. This suggests that the lower purity dopants could be a potential contamination sources for these impurities and higher purity could be achieved using high purity dopants and Cd, Te elements.
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Figure 4, (a) Comparison of total impurity concentration between HPB grown CdTe:Sb crystals and various doped CdTe grown in sealed quartz ampoules in VB technique. Purity of crystals which has produced high performing single crystal solar cell and gamma ray detectors are also shown. (b) Impurity concentrations in the two HPB-grown and a VB-grown CdTe:Sb crystals. Both undoped and doped HPB grown feedstock have been used to produce solar cells by CdTe vapor transport deposition (VTD) [9] over a wide range of conditions with efficiencies ranging from 13–18%. The deposition was made on a glass substrate coated with transparent conductive oxide (TCO). The CdSeTe:As layer was deposited by close space sublimation whereas the CdTe;As layer was deposited by vapor transport deposition (VTD) technique. The performance using HPB grown feedstock is comparable to that produced by the Bridgman method. However, HPB provides more flexibility in modifying dopant levels to optimize performance [20] as well as a path to larger scale and lower costs.
For example, HPB boules were synthesized from the
elements with As and then used as source material to grow thin CdTe films by VTD. Secondary ion mass spectrometry (SIMS) indicates As incorporation levels of 1016–1018 cm-3 can be achieved (e.g. Fig 5) in both CdSeTe and CdTe. By varying the As levels in the feedstock or deposition
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conditions, As incorporation levels can be increased or decreased, and hole density as high as 1017 cm-3 can be achieved. At the same time, minority carrier lifetime up to 40 ns is obtained from the device shown in figure 5(d), which is commensurate with state of the art CdTe thin film devices. The methods can be applied similarly for different dopants such as Sb and In, and useful for other applications.
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(b)
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Figure 5, (a) Device structure of the thin film CdTe/CdSeTe doped with As. (b) SIMS depth profile of As incorporation. Both CdTe:As and CdSeTe:As source material were prepared by HPB. (c) Hole density obtained by C-V measurement and (d) Life-time measured by 2 photon excitation.
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4. Conclusion The results indicate that the HPB technique is an efficient melt-growth process that can maintain good control of purity and stoichiometry. CdTe with grain size comparable to ingots formed in sealed quartz ampoules by the MVB technique were synthesized at fast growth rates by HPB and coupled with Se alloying and effective II-VI semiconductor doping. The HPB capability to process tens of kilograms of material in a single batch offers avenues for enhancing material properties, scaling to manufacturing, and reducing costs for electro-optical applications. Acknowledgement The authors would like to thank Muad Saleh for performing the XRD measurements. The authors acknowledge financial support from U.S. Department of energy, Office of Energy Efficiency and Renewable Energy under Contracts DE-EE0007537 and DE-AC36-08GO28308. Tawfeeq K. Al-Hamdi would like to thank the Iraqi Ministry of Higher Education and Scientific Research and Mustansiriya University-College of Engineering for sponsoring his Ph.D. study in Washington State University (WSU)- School of Mechanical and Materials Engineering.
References 1. 2. 3. 4. 5. 6.
Takahashi, T. and S. Watanabe, Recent progress in CdTe and CdZnTe detectors. IEEE Transactions on Nuclear Science, 2001. 48(4): p. 950-959. Roy, U.N., et al., Growth and characterization of detector-grade CdMnTe by the vertical Bridgman technique. AIP Advances, 2018. 8(10): p. 105012. Roy, U.N., et al., Growth and characterization of Cd1-xZnxSeyTe1-y for radiation detector applications (Conference Presentation). SPIE Optical Engineering + Applications. Vol. 9968. 2016: SPIE. 1. Munshi, A.H., et al., Polycrystalline CdSeTe/CdTe Absorber Cells With 28 mA/cm2 Short-Circuit Current. IEEE Journal of Photovoltaics, 2018. 8(1): p. 310-314. Wu, X., High-efficiency polycrystalline CdTe thin-film solar cells. Solar Energy, 2004. 77(6): p. 803814. Iniewski, K., CZT detector technology for medical imaging. Journal of Instrumentation, 2014. 9(11): p. C11001-C11001. 12
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Zappettini, A. et. al, A new process for synthesizing high-purity stoichiometric cadmium telluride, Journal of Crystal Growth 214/215 (2000), pp. 14-18. Triboulet, R., Fundamentals of the CdTe synthesis. Journal of Alloys and Compounds, 2004. 371(1): p. 67-71. McCandless, B.E., et al., Overcoming Carrier Concentration Limits in Polycrystalline CdTe Thin Films with In Situ Doping. Scientific Reports, 2018. 8(1): p. 14519. Danielson, A., et al. Doping CdTe Absorber Cells using Group V Elements. in 2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC). 2018. Metzger, W., et al., Exceeding 20% efficiency with in-situ group V doping in polycrystalline CdTe solar cells, Nature Energy, 2019, 4, p. 837. Szeles, C. and E.E. Eissler, Advances in the crystal growth and device fabrication technology of CdZnTe room temperature radiation detectors. IEEE Transactions on Nuclear Science, Vol. 51, No. 3, 2004, pp. 1242-1249. Burst, J.M., et al., CdTe solar cells with open-circuit voltage breaking the 1 V barrier. Nature Energy, 2016. 1: p. 16015. A. Nagaoka, Kyu-Bum Han, S. Misra, T. Wilenski, T. Sparks and M. Scarpulla, Growth and characterization of Arsenic doped CdTe single crystals grown by Cd-solvent traveling-heater method, Journal of Crystal Growth 467 (2017) 6-11. C. Su, S. L. Lehoczky, B. Raghothamachar, and M. Dudley, Crystal growth and characterization of CdTe grown by vertical gradient freeze, Materials Science and Engineering B 147 (2008) 35–42. Kaufmann, U., J. Windscheif, and G. Brunthaler, Identification of the isolated deep Ni acceptor in CdTe and ZnTe: comparison with isomorphous systems. Journal of Physics C: Solid State Physics, 1984. 17(34): p. 6169-6176. Rzepka, E., et al., Deep centres for optical processing in CdTe. Materials Science and Engineering: B, 1993. 16(1): p. 262-267. Balcioglu, A., R.K. Ahrenkiel, and F. Hasoon, Deep-level impurities in CdTe/CdS thin-film solar cells. Journal of Applied Physics, 2000. 88(12): p. 7175-7178. Jr., R.C.B. and D.E. Cooper, Detection of dilute iron impurities in CdTe. Applied Physics Letters, 1988. 53(16): p. 1521-1523. A. Kanevce, M.O.R., T. M. Barnes, S. A. Jensen, and W. K. Metzger, The roles of carrier concentration and interface, bulk, and grain-boundary recombination for 25% efficient CdTe solar cells. Journal of Applied Physics, 2017. 121: p. 214506.
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Highlights 1. High pressure Bridgman technique is suitable to synthesize CdTe from elemental sources 2. Highly volatile dopants can be efficiently incorporated into melt grown CdTe under high pressure 3. High purity level in CdTe is achievable in scalable HPB melt growth process
Author contributions S. Swain, T. Al-Hamdi, S. Mcpherson, J. Jennings, C. Szeles, K. Lynn- Performed crystal growth in high pressure Bridgman furnace J. Duenow, X-Zheng, D. S. Albin, T. Ablekim, M. Amarasinghe, A. Ferguson, W. Metzger- Performed the thin film deposition, cell fabrication and measurements S. Swain, W. K. Metzger- Wrote the manuscript
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