WANG ET AL. VOL. 8 ’ NO. 5 ’ 4859–4865 ’ 2014 www.acsnano.org 4859 April 03, 2014 C 2014 American Chemical Society Role of Ga Vacancy on a Multilayer GaTe Phototransistor Zhenxing Wang, † Kai Xu, † Yuanchang Li, Xueying Zhan, Muhammad Safdar, Qisheng Wang, Fengmei Wang, and Jun He * National Center for Nanoscience and Technology, Beijing 100190, People's Republic of China. † These authors contributed equally to this work. A s an emerging class of new material, two-dimensional (2D) layered mate- rials have attracted considerable in- terest in the past decade since graphene was discovered in 2004. 1À3 In contrast to graphene, the presence of an inherent band gap allows 2D semiconductors to be highly promising building blocks for high-performance electronic and optoelectronic applications. 4À6 Moreover, compared to traditional semi- conductor materials, such as Si and IIIÀV group materials, 2D layered semiconduct- ing materials exhibit three important fea- tures: (1) their ultrathin thickness (atomic level) is beneficial for efficient electrostatic gating and high degree of vertical integra- tion for field-effect transistor (FET); (2) the intrinsic surface free of dangling bonds effectively reduces the surface charge trap- ping states and roughness scattering, thus obtaining high channel mobility; (3) owing to the nature of 2D structures, by employing current thin film micromanufacturing tech- niques, they are relatively easily fabricated into complex devices. As a representative, MoS 2 is the most widely investigated 2D layered semiconducting material. Single- layer MoS 2 FET has demonstrated excellent performance, for instance, relatively high channel mobility (∼200 cm 2 V À1 s À1 ) and high on/off switch ratio (10 8 ). 7 Recently, Kis et al. has improved the field-effect mobility to exceed 1000 cm 2 V À1 s À1 . 8 Encouraged by the achievements in MoS 2 , various other 2D layered semicon- ductors such as the family of IIIÀVI com- pounds (GaS, GaSe, and InSe) have been studied. 9À11 Gallium telluride (GaTe) is an- other important 2D layered IIIÀVI semicon- ductor with a direct band gap around 1.7 eV at room temperature in bulk form. 12,13 However, the properties and applications of GaTe are not well-known. This is most likely due to its complicated crystal struc- ture and possible high defect density. 12À14 Strikingly different with the hexagonal structure of GaS and GaSe, GaTe crystallizes into the more complicated and less sym- metric monoclinic structure with the space group C 2h 3 , as shown in Figure 1a. Espe- cially, there are two kinds of GaÀGa bonds in one single layer: two-thirds are perpendi- cular to the layer and one-third lie in the layer. As a result, there is only a two-fold rotational symmetry along the b axis and no rotational symmetry perpendicular to the (20À1) layer plane (Figure 1b). To date, most discovered 2D layered materials, such as graphene, MoS 2 , WS 2 , WSe 2 , GaS, GaSe, and so on, almost have relatively high * Address correspondence to hej@nanoctr.cn. Received for review February 9, 2014 and accepted April 3, 2014. Published online 10.1021/nn500782n ABSTRACT We report a high-performance field-effect transistor (FET) and phototransistor based on back-gated multilayer GaTe nanosheets. Through both electrical transport measurements at variable temperatures and first-principles calculations, we find Ga ion vacancy is the critical factor that causes high off-state current, low on/off ratio, and large hysteresis of GaTe FET at room temperature. By suppressing thermally activated Ga vacancy defects at liquid nitrogen temperature, a GaTe nanosheet FET with on/off ratio of ∼10 5 ,off- state current of ∼10 À12 A, and negligible gate hysteresis is successfully demonstrated. Furthermore, a GaTe phototransistor with high photogain above 2000 and high responsivity over 800 AW À1 is achieved, as well. Our findings are of scientific importance to understand the physical nature of intrinsic GaTe transistor performance degradation and also technical significance to unlock the hurdle for practical applications of GaTe transistors in the future. KEYWORDS: gallium telluride . transistor . two-dimensional . layered materials . vacancy ARTICLE