Quantitative Determination of Contradictory Bandgap Values of Bulk PdSe₂ from Electrical Transport Properties

Abstract

2D PdSe2, a Group 10 noble metal dichalcogenide, has been reported to have a strong thickness‐dependent bandgap energy, ranging from ≈1.6 eV (monolayer) to ≈0.05 eV (bulk) and a high photoresponsivity for bulk samples in the far infrared wavelength range of 10.6 μm. However, a middle bandgap of ≈0.5 eV has been contradictorily reported for bulk PdSe2 via optical absorption measurements. In this study, detailed electrical transport measurements are conducted to solve this contradiction. The key difference between narrow gap and middle gap semiconductors is the contribution of a depletion layer to the transfer characteristics. Hall measurements reveal intrinsic p‐type carrier densities of ≈1.9 × 1018 cm−3 at 300 K and the contribution of the depletion layer to the transfer characteristics for bulk PdSe2, suggesting a middle bandgap. Moreover, the maximum depletion width (WDm) is determined from top‐ and back‐gate coupling in dual gate transistors to be ≈17–18 nm. Based on the WDm – acceptor density diagram, the bandgap of bulk PdSe2 is quantitatively estimated to be ≈0.3 eV. Although this is not a desirable result from the viewpoint of far infrared material, it helps us to correctly understand the mechanism for the photoresponse of PdSe2.

Is the Bandgap of Bulk PdSe2 Located Truly in the Far‐Infrared Region? Determination by Fourier‐Transform Photocurrent Spectroscopy

“…As shown in Figure a, two terminal bulk PdSe 2 devices for photocurrent measurements were prepared on an SiO 2 /Si substrate by the mechanical exfoliation of PdSe 2 crystals; these crystals have been previously characterized to possess an orthorhombic structure with the space group Pbca [ 53 ] by powder X‐ray diffraction (XRD), Raman spectroscopy, and X‐ray photoelectron spectroscopy (XPS). [ 31 ] The thickness dependence of E G is reported to be gradual from a monolayer to a 20 nm‐thick layer and then becomes constant for layers with a thickness ( t ) > 20 nm. [ 23 ] Therefore, 54 nm‐thick PdSe 2 confirmed by atomic force microscopy (AFM) was selected here to investigate E G for bulk PdSe 2 .…”

“…Moreover, the overestimation of E G for degenerately doped semiconductors has been well known as the Moss–Burstein effect, [ 3 ] where the relation between the increase in E G (Δ E G ) and the carrier concentration ( n ) is given by . [ 58 ] As the hole concentration for bulk PdSe 2 is 1.9 × 10 18 cm −3 at RT [ 31 ] and m * is 0.28, [ 12 ] Δ E G can be estimated to be ≈1.24 × 10 −3 eV at RT, which is two orders smaller than E G . Therefore, the contribution of the carrier concentration can be neglected.…”

“…PdSe 2 is recognized as a new FIR material in the 2D research community. [27][28][29] Recently, it has been studied very intensively due to its characteristics, such as high mobility, [30][31][32] strong in-plane anisotropy, [30,[33][34][35][36] air stability, [37][38][39][40] nonlinear optical properties, [41,42] and thermoelectric properties. [43,44] However, when the absorption measurements of bulk PdSe 2 are reexamined, [12,45] it was realized that zero bandgap was claimed by the extrapolation exceeding 1 eV, suggesting the large uncertainty in E G determination.…”

“…Since HgCdTe [ 25,26 ] and graphene are the only materials that have a bandgap in the FIR region thus far, PdSe 2 is recognized as a new FIR material in the 2D research community. [ 27–29 ] Recently, it has been studied very intensively due to its characteristics, such as high mobility, [ 30–32 ] strong in‐plane anisotropy, [ 30,33–36 ] air stability, [ 37–40 ] nonlinear optical properties, [ 41,42 ] and thermoelectric properties. [ 43,44 ]…”

“…Therefore, based on the carrier transport characteristics, not by the absorption measurement of the small flake, we have revealed that E G is ≈0.3 eV. [ 31 ] However, determining E G by multifaceted experimental considerations, especially optical absorption measurements, is still essential.…”