Fast-track communicationGate-tunable transport characteristics of Bi2S3 nanowire transistors
Introduction
Semiconducting nanowire field effect transistors (NWFETs) have been a staple of research in low dimensional electronics for the efforts of finding a suitable means of scaling down current technology [[1], [2], [3]]. Nanowires have been envisioned to fulfill roles in thermoelectrics [4,5], optoelectronics [6,7], and as various sensors [8,9]. Nanowires are advantageous for these technologies because of their superior properties compared to planar films; since nanowires have exhibited lower noise intensity [10], better electrostatic control [11] and drastically lower thermal conductivity due to phonon confinement which is beneficial for thermoelectrics [12]. For optoelectronics, it has been shown that the light trapping properties of nanowires make them more efficient than thin films [13]. Also, incorporating III - V materials with existing silicon technology has been an ongoing challenge due to lattice mismatch, but nanowires have changed that with epitaxial growth on silicon [14]. However, the high surface area-to-volume ratio of semiconducting nanowires means that they are at constant risk of having surface defects which significantly hinder their mobility compared to their bulk counterparts [15,16]. In addition, the growth of these nanowires followed by the fabrication of devices with parameters such as annealing time and choice of contact metals have a dramatic impact on the performance of these devices [17]. Hence, understanding defect behavior that often plague device performances are a necessity.
Layered semiconducting chalcogenides, especially bismuth chalcogenides, are a family of materials that have been drawing increased attention as low dimensional structures due to their vast array of interesting physical properties. Two prominent members, bismuth selenide, Bi2Se3 and bismuth telluride, Bi2Te3 are topological insulators [[18], [19], [20]], and are already established as good thermoelectric materials [21]. However, Bi2S3 lacks the toxicity of those two materials and the fact that sulfur vacancies could be potentially tuned in this material makes it worthy of exploring for electronic, thermoelectric and optical applications. Bi2S3, a prominent family member with an orthorhombic crystal structure, has many diverse and useful qualities such as a small band gap of 1.3–1.7 eV [22,23], high bulk mobility [24], high Seebeck coefficient [25], and low thermal conductivity [24,25]. Due to these distinct physical properties, Bi2S3 has been incorporated as the active material in optical [26,27], electronic [28,29] and thermoelectric technologies [30].
Additionally, transition-metal sulfides are attractive candidates for defect engineering of sulfur vacancies. Sulfur vacancies can impact the band structure of a material and thereby alter the electronic and optical properties [31]. For example, the formation of mid-gap states from these defects has been seen in other sulfide materials such as in MoS2 and WS2 [32,33] and has been attributed to the low luminescence quantum yield. The vacancies have also been shown to act as adsorption sites for gases which can lead to modulated electronic performance in the form of increased or decreased conductivity and field effect mobility depending on the specific gas [34]. Recently, calculations have shown that native sulfur vacancies can likewise produce mid-gap states in the Bi2S3 band structure which act as charge traps [[35], [36], [37]]. In previous works, it was experimentally shown that Bi2S3 optoelectronic devices had their performance and quantum yield hindered by what qualitatively seemed to be charge trapping which affects electron and hole recombination [38,39]. The impact of this altered band structure on the transport properties for the most fundamental building block of electronics, the transistor, have not been realized. In this work we show a systematic study of the transport characteristics in Bi2S3 nanowire transistors and present supporting evidence for the mid-gap states induced from vacancies. Specifically, we characterized the charge transport properties of Bi2S3 nanowires over a wide temperature range and developed a deeper understanding of the charge transport mechanisms in Bi2S3 single nanowire field effect transistors via transport and noise spectroscopy studies. Noise spectroscopy is a method to probe the intrinsic fluctuations of a system to obtain an understanding of the microscopic conduction mechanisms, which normally cannot be obtained through standard transport characterization [40]. Our current work sets the stage for many new works to be explored, from deeper noise spectroscopy studies to defect engineering to tune these vacancies and the electronic properties of Bi2S3 for future applications.
Section snippets
Nanowire growth & device fabrication
The Bi2S3 nanowires with a mean diameter of 36 nm were synthesized through a hydrothermal method, the details of which can be found elsewhere [41]. The orthorhombic structure is depicted in Fig. 1 (A). Bi2S3 possesses a highly anisotropic lamellar crystal structure that consists of ribbon-like [Bi4S6] cages linked together by the intermolecular interaction between Bi and S atoms. These ribbon-like [Bi4S6] cages, which are oriented parallel to the c-axis, dictate the growth direction in Bi2S3
Transistor characterization
Transistor characteristics were studied at several drains-source biases as well as at several temperatures. All nanowires show n-type conduction which has been previously attributed to sulfur vacancies in other Bi2S3 works [44]. Other n-type sulfide materials such as MoS2 have shown the same origin for their n-type conductance [45]. Gate voltage sweeps at various temperatures are shown in Fig. 2 (C). As the temperature is increased, the threshold voltage to turn the device ON decreases and the
Conclusion
In conclusion, Bi2S3 nanowire back gated field effect transistors have been characterized and their transistor parameters such as, mobility and ON/OFF were studied. Mobility is shown to be dependent on both electron-lattice scattering as well as defect/impurity scattering as temperature and drain-source bias are modified. The overall values range from 0.041 to 2.58 cm2/V s. The ON/OFF ratio ranged from 2 to 5 orders which at room temperature became roughly bias independent due to a large
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