Molecular Dopant‐Dependent Charge Transport in Surface‐Charge‐Transfer‐Doped Tungsten Diselenide Field Effect Transistors

Abstract

The controllability of carrier density and major carrier type of transition metal dichalcogenides(TMDCs) is critical for electronic and optoelectronic device applications. To utilize doping in TMDC devices, it is important to understand the role of dopants in charge transport properties of TMDCs. Here, the effects of molecular doping on the charge transport properties of tungsten diselenide (WSe2) are investigated using three p‐type molecular dopants, 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ), tris(4‐bromophenyl)ammoniumyl hexachloroantimonate (magic blue), and molybdenum tris(1,2‐bis(trifluoromethyl)ethane‐1,2‐dithiolene) (Mo(tfd‐COCF3)3). The temperature‐dependent transport measurements show that the dopant counterions on WSe2 surface can induce Coulomb scattering in WSe2 channel and the degree of scattering is significantly dependent on the dopant. Furthermore, the quantitative analysis revealed that the amount of charge transfer between WSe2 and dopants is related to not only doping density, but also the contribution of each dopant ion toward Coulomb scattering. The first‐principles density functional theory calculations show that the amount of charge transfer is mainly determined by intrinsic properties of the dopant molecules such as relative frontier orbital positions and their spin configurations. The authors’ systematic investigation of the charge transport of doped TMDCs will be directly relevant for pursuing molecular routes for efficient and controllable doping in TMDC nanoelectronic devices.

Reduced dopant-induced scattering in remote charge-transfer-doped MoS 2 field-effect transistors

“…4, the temperature-dependent mobilities of pristine MoS 2 FETs, and directly and remotely doped MoS 2 FETs increased as the temperature decreased because of suppressed phonon scattering ( 51 ). According to Matthiessen’s rule, the mobility of the channel can be written asμ4ppfalse(Tfalse)=(1μC Tα+1μphTβ)−1where T , μ c , μ ph , and α and β denote temperature, charged impurity scattering–limited mobility, and phonon-limited mobility at the zero-temperature limit, and their exponents, respectively ( 40 , 48 , 52 , 53 ). In this analysis, we assumed that the scattering sources, excluding charged impurity and phonon, such as intrinsic defects and the roughness of the substrate, were negligible because the charged impurity and phonon scattering are the most dominant mechanisms in the charge transport of MoS 2 , as demonstrated previously ( 48 , 51 ).…”

“…In the model, the charged impurity scattering rate that is dependent on the charged impurity scattering potential (ϕ scr ) determines the charged impurity–limited mobility of MoS 2 FETs. While this model is successful in describing charged impurity scattering in a directly doped TMDC channel where the dopant ions are present on the surface of TMDC ( 40 ), the model has to be corrected for the remotely doped channel because of the spatial separation introduced by the h-BN interlayer. Considering the spatial separation, which places the charged impurities at a finite distance from the channel, the scattering potential in the model should be modified as the following equation ( 59 )ϕqremote=e−italicqdϕqscrwhere ϕqremote and ϕqscr are charged impurity scattering potential in the case of remote doping and direct doping, respectively, d is the h-BN thickness, and q is the scattering vector, defined as the magnitude of the difference in the scattered and initial wave vectors.…”

“…where T,  c ,  ph , and  and  denote temperature, charged impurity scattering-limited mobility, and phonon-limited mobility at the zerotemperature limit, and their exponents, respectively (40,48,52,53).…”

Enhanced Thermoelectric Power Factor in Carrier‐Type‐Controlled Platinum Diselenide Nanosheets by Molecular Charge‐Transfer Doping

“…Surface charge-transfer doping (SCTD) is one of the most effective doping methods to alter the carrier concentrations in TMDCs. [14][15][16][17][18][19][20][21] This method proceeds through the charge transfer between the host semiconductor and dopants unlike substitutional doping, which involves the replacement of constituent transition metal elements in the structure with those with a different number of valence electrons. Therefore, SCTD is relatively nondestructive because the dopants can be deposited on the host semiconductor surface through postsynthesis treatments.…”

“…In particular, molecular SCTD is a facile solution phase doping route, which allows the design of a diverse range of molecular structures with highly tunable carrier types (both n-and p-type dopings) and doping strengths, thereby achieving a wide range of doping levels in various materials, including TMDCs. [14,16,17,19,21] However, despite the immense potential of this modification strategy, the effect of doping on the thermoelectric properties of TMDCs has not been studied extensively in literature. [22][23][24] Here, n-and p-type-doped TMDC nanosheets from the same parental PtSe 2 crystal were synthesized through the SCTD approach.…”