Letter pubs.acs.org/NanoLett
Giant Piezoresistive On/Off Ratios in Rare-Earth Chalcogenide Thin Films Enabling Nanomechanical Switching M. Copel,* M. A. Kuroda, M. S. Gordon, X.-H. Liu, S. S. Mahajan, G. J. Martyna, N. Moumen, C. Armstrong, S. M. Rossnagel, T. M. Shaw, P. M. Solomon, T. N. Theis, J. J. Yurkas, Y. Zhu, and D. M. Newns IBM Research Division, T. J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598, United States S Supporting Information *
ABSTRACT: Sophisticated microelectromechanical systems for device and sensor applications have flourished in the past decade. These devices exploit piezoelectric, capacitive, and piezoresistive effects, and coupling between them. However, high-performance piezoresistivity (as defined by on/off ratio) has primarily been observed in macroscopic single crystals.1 In this Letter, we show for the first time that rare-earth monochalcogenides in thin film form can modulate a current by more than 1000 times due to a pressure-induced insulator to metal transition. Furthermore, films as thin as 8 nm show a piezoresistive response. The combination of high performance and scalability make these promising candidates for nanoscale applications, such as the recently proposed piezoelectronic transistor (PET).2,3 The PET would mechanically couple a piezoelectric thin film with a piezoresistive switching layer, potentially scaling to higher speeds and lower powers than today’s complementary metal−oxide−semiconductor technology. KEYWORDS: Piezoresistance, piezotronic, chalcogenide, electrical transport, nanoelectromechanical systems (NEMS), MEMS that the response in thin films is comparable to bulk samples. Note that the PET in this letter is radically different from the piezotronic devices used for energy harvesting and strain sensing. In those devices, an external strain creates charge across a PE, gating carriers in a semiconductor channel.8−10 In the application discussed here, the actuator and switch are mechanically coupled, but are electrically distinct, save sharing a common metallization layer. Numerous materials show piezoresistivity, however consideration of their properties limits the scope of acceptable materials (see Table 1). As mentioned above, logic gates demand both a high on/off ratio and a large piezoresistive gauge, defined as
A
logic device consists of two components, an actuator and a switch. In the case of a field effect transistor (FET), the gate is the actuator and the semiconductor channel is the switch. In a PET, a piezoelectric (PE) film is the actuator, and a piezoresistive (PR) layer is the switch2,3 (Figure 1a). Simulations of device performance using bulk material properties are promising. With a device built at 11 nm lithography scale, switching speeds of 4 GHz could be attained at power levels 50× lower than similarly scaled complementary metal−oxide−semiconductor (FinFET) technology, and the PET continues to higher speeds and lower powers when scaled to still smaller dimensions. Relaxor PEs have the performance to drive a PET.4,5 Furthermore, functional piezoelectrics at nanoscale dimensions have been reported.6,7 As for the PR, bulk, single crystal SmSe samples1 show more than adequate properties. The 104× resistance modulation required in logic applications can be achieved with modest pressures (
