Pressure-driven p-n switch in CuFeS2 – Drs. Ting Wen, Yonggang Wang & Wenge Yang

NOVEMBER 29, 2018

A new work from a group of scientists co-led by Drs. Ting Wen, Wenge Yang and Yonggang Wang of HPSTAR reported a pressure-driven reversible switching between n- and p-type conduction in chalcopyrite CuFeS₂. The successful realization of an abrupt conduction type switch in transition metal semiconductors under high pressure paves the way to novel pressure-responsive switching devices. This work is published online in recent J. Am. Chem. Soc.

Materials with phase change or physical property change abilities are some of the most promising candidates for switch or memory applications. Recently, an emerging class of inorganic semiconductors with fascinating switching ability between p- and n-type conduction upon temperature change has been discovered, which shows great potential in transistor or memory applications. Up to now, such temperature-dependent p-n switching materials are limited in only a few Ag-based compounds, all of which show dramatic and reversible p-n type conduction switching upon heating as evidenced by the Seebeck coefficient analyses.

Pressure, as an efficient external stimulus that has driven collective phenomena such as spin crossover and Mott transition is also expected to initialize a conduction type switching in transition metal based semiconductors. In fact, pressure-induced p-n switching has been observed in several distinct systems including elements, metal chalcogenides, and transition metal oxides. However, the pressure-driven p-n switching materials are still far away from meeting the requirements of practical applications if compared with the temperature-dependent counterparts. A reversible and abrupt conduction type change within a narrow pressure interval is required.

Mineral chalcopyrite CuFeS₂ is a good candidate to achieve such pressure-driven p-n switching phenomenon. First of all, CuFeS₂ undergoes a phase transition around 8 GPa with symmetry breakdown from space group I-42d to I-4 accompanying with the shrinkage of FeS₄ tetrahedra. This ensures a surprising semiconductor-to-semiconductor transition instead of pressure-driven metallization. A high-to-low spin-crossover of Fe²⁺ (S = 2 to S = 0) is manifested along with this phase transition as evidenced by X-ray emission spectra. Both photocurrent and Hall coefficient measurements confirm that CuFeS₂ undergoes a reversible pressure-driven p-n type conduction switching accompanying with the structural phase transition. They also confirmed the absence of cationic charge transfer between copper and iron during the phase transition by both X-ray absorption near-edge spectra (Cu/Fe, K-edge) and total-fluorescence-yield X-ray absorption spectra (Fe, K-edge) results. The valence distribution maintains Cu²⁺Fe²⁺S₂ in the high pressure phase.

“CuFeS₂ is the first semiconductor with an abrupt and reversible pressure-driven p-n switching phenomenon”, said Dr. Wenge Yang. “Our original intention is to study the possible charge transfer process and metallization of CuFeS₂ under high pressure. Unfortunately, things are always not going the way they imagined, but accident is just the source of discovery.” “We are developing many new in situ measurement platforms for material design under extreme conditions”. said Ting Wen, the leading author of this work. “In our experiments, for example, the self-made photocurrent system is the key to the discovery of pressure-driven p-n switching material”. They also believe their finding can pave the way to novel pressure-responsive switching devices.

Caption: Pressure-driven n- to p-type conduction switching in CuFeS₂ as evidenced by photocurrent and Hall resistance measurements.

双态转换半导体材料被广泛应用于开关元件和信息存储，是现代电子科技发展的基础。其中，材料在外界刺激下状态转换可以包括结构相变、磁性、导电性和光学性质等，控制双态转换的手段则多为光照、温度和电场。在由北京高压科学研究中心的王永刚和杨文革研究员负责的一项研究中，首次在黄铜矿CuFeS2中实现了陡峭和可逆的压致p-n转换，有望应用于压力敏感的转换或开关元件中，以及利用应力实现特殊构型的p-n结构。伴随着结构相变、二价铁的高低自旋变化和半导体到半导体的转变，CuFeS2在8 GPa附近发生了载流子类型的转换。证据包括原位高压光电流测试和霍尔系数测试结果。该发现为新型压致转换材料的结构设计提供了一种新的思路。该研究结果发表于近期的 J. Am. Chem. Soc.。