Majority Carrier Properties of Single Crystal Cu _{2− x} ZnSnSe ₄ with Varying Copper Composition

Journal Article · Tue Jul 16 00:00:00 EDT 2019 · Physica Status Solidi B. Basic Solid State Physics

DOI:https://doi.org/10.1002/pssb.201900180· OSTI ID:1542679

McCandless, Brian E.  ^[1]; Bishop, Doug ^[2]; Lloyd, Michael ^[1]

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1. Institute of Energy Conversion University of Delaware 451 Wyoming Road Newark DE 19716 USA
2. IBM TJ Watson Research Center 1101 Kitchawan Rd Yorktown Heights NY 10598 USA

The dependence of carrier concentration on copper content in Cu _{2− x} ZnSnSe ₄ single crystals, grown without fluxing agents, is presented over the composition range Cu/(Zn + Sn) of 0.77–0.93 and Zn/Sn from 1.3 (low Cu content) to 1.3 (high Cu content). Single phase mono‐crystals with facets up to 5 mm are synthesized from high‐purity elements and validated using Laue diffraction. A direct optical band gap of 0.98 eV is observed for a crystal with Cu/(Zn + Sn) = 0.85. Van der Pauw–Hall resistance analysis shows decreasing hole concentration, from 10 ¹⁹  cm ⁻³ to 2 × 10 ¹⁵  cm ⁻³ , with decreasing copper content over the composition range. Hole mobility varies from 50 cm ²  V ⁻¹  s ⁻¹ to 160 cm ²  V ⁻¹  s ⁻¹ , with no dependence on copper concentration. A linear temperature dependence of the Seebeck coefficient is found for all crystals. The Seebeck coefficient–carrier concentration dependence is used to determine the valence band density of states, N _V , and corresponding effective mass, μ _h *. Crystals with Cu/(Zn + Sn) < 0.9 have N _V  = 4 × 10 ¹⁸  cm ⁻³ and μ _h * = 0.4 m ₀ while crystals with elevated copper concentration, Cu/(Zn + Sn) > 0.9, exhibit higher density of states, N _V  = 3 × 10 ¹⁹  cm ⁻³ and hole effective mass μ _h * = 0.9 m ₀ . The increased valence band density of states at elevated copper composition is consistent with more Cu–Zn exchange sites contributing to bulk defects and band edge fluctuations. Temperature‐dependent conductivity measurements indicate a transition from metallic‐type to semiconductor‐type conduction at Cu/(Zn + Sn) < 0.9. At lower copper content, for a crystal with Cu/(Zn + Sn) = 0.83 and p  = 10 ¹⁷  cm ⁻³ the activation energy for intrinsic conduction is 60 meV. This behavior is attributed to the copper vacancy defect, V _Cu , and is comparable to reported values obtained in thin films and by first‐principles predictions.

• Sponsoring Organization: USDOE
• Grant/Contract Number: DE‐EE0006334
• OSTI ID: 1542679
• Journal Information: Physica Status Solidi B. Basic Solid State Physics, Journal Name: Physica Status Solidi B. Basic Solid State Physics Vol. 256 Journal Issue: 11; ISSN 0370-1972
• Publisher: Wiley Blackwell (John Wiley & Sons)
• Country of Publication: Germany
• Language: English