Electrical properties and band alignments of Sb2Te3/Si heterojunctions, low-barrier Sb2Te3/n-Si and high-barrier Sb2Te3/p-Si junctions

We investigated the electrical junction properties of the layered Sb2Te3 film formed on Si substrates. The current−voltage characteristics of the Sb2Te3/n-Si heterojunction showed an ohmic properties, whereas the Sb2Te3/p-Si heterojunction exhibited rectifying properties with a high barrier height of 0.77 eV. The capacitance−voltage characteristics of MOS capacitors with the Sb2Te3 electrode indicated an effective work function of 4.44 eV for the Sb2Te3 film. These findings suggest that the Sb2Te3/Si heterostructure possesses a low conduction band offset, as inferred from the temperature dependence of the current−voltage characteristics of the Sb2Te3/n-Si.


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emiconductor heterostructures offer enhanced functionality and improved performance in electric and optoelectronic devices, such as transistors, LED, lasers, and solar cells.These heterostructures are typically designed by selecting materials with similar lattice symmetry and constants, such as SiGe/Si, [1][2][3][4][5] AlGaN/GaN, [6][7][8][9] while also ensuring precise control over the abrupt interfaces. Tis approach is instrumental in improving device performance; however, there are limitations in the choice of materials to be used in heterostructures. To tanscend these constraints associated with conventional heterostructures, one promising avenue is the utilization of van der Waals (vdW) layered materials characterized by a dangling-bond-free surface.Prominent examples of such materials include transitionmetal dichalcogenides, transition-metal oxides, and pnictogen chalcogenides.[10][11][12][13][14][15] Consequently, the development of heterostructures utilizing vdW layered materials has gained substantial attention in recent research.
In a previous study, we demonstrated the vdW stacking of antimony telluride (Sb 2 Te 3 ) on molybdenum disulfide (MoS 2 ), 16) and showed a substantial enhancement in the device performance of monolayer MoS 2 n-type FETs with Sb 2 Te 3 source/drain contacts, which was resulted from a significant reduction of contact resistance.This result suggests that the conduction band edges of Sb 2 Te 3 and MoS 2 are closely aligned.This finding raises the possibility that Sb 2 Te 3 could form a low conduction band offset on Si, given the proximity of the conduction band edge of Si to that of MoS 2 .][19] While these photodiodes showed rectification behavior in their current−voltage (I−V ) curves and exhibited photodetector characteristics, a comprehensive elucidation of the electrical properties of this heterojunction remained undisclosed.
In the present study, we demonstrate the low-barrier heterojunction formed by Sb 2 Te 3 on an n-Si substrate and the high-barrier heterojunction formed by Sb 2 Te 3 on a p-type Si (p-Si) substrate through the I−V measurements.Furthermore, we delve into the band aliment of the Sb 2 Te 3 /Si heterojunctions by estimating the effective work function (eWF) of Sb 2 Te 3 , as determined from capacitance−voltage (C−V ) measurements of MOS capacitors with Sb 2 Te 3 electrodes.
The Sb 2 Te 3 film used in this study were prepared by RF magnetron sputtering with a target of Te-rich Sb 33.3 Te 66.7 alloy onto a substrate kept at 230 °C. 20)The substrates used were n-and p-type (100) Si with a resistivity of ∼2 Ω•cm and ∼5 Ω•cm, respectively.To prepare the Sb 2 Te 3 /Si heterojunction with a clean interface, the Si substrate was treated in dilute hydrofluoric (HF) acid (0.5%), and the Sb 2 Te 3 film of 20 nm thickness and the W electrodes of 30 nm thickness were successively formed in the same sputtering system without braking the vacuum (W/Sb 2 Te 3 /Si).Note that the Sb 2 Te 3 film was deposited under the condition where the Si surface was passivated by hydrogen, thereby suppressing the formation of interface states.To investigate the eWF of Sb 2 Te 3 , we prepared MOS capacitors with SiO 2 thickness in the range from 24−103 nm (W/Sb 2 Te 3 /SiO 2 /n-Si).
Figure 1 shows a cross-sectional transmission electron microscopy (TEM) image of the Sb 2 Te 3 /Si heterojunction.A layered structure in the Sb 2 Te 3 film was formed on the cleaned Si surface without distinct interfacial layers.Although no obvious grain boundaries were observed in this TEM image, the lateral grain size of Sb 2 Te 3 was estimated to be less than around 140 nm by electron backscatter diffraction analysis as shown in Fig. S1.On the other hand, high c-axis oriented Sb 2 Te 3 was confirmed by X-ray diffraction analysis in Fig. S2.
We performed I−V measurements for the W/Sb 2 Te 3 /Si heterojunctions and the W/Si direct junctions at RT, as shown in Fig. 2(a).The I−V curves for the W/Si direct junction showed rectifying characteristics and revealed an electron Schottky barrier height of 0.67 ± 0.14 eV and a hole Schottky barrier height of 0.41 ± 0.13 eV.These values were calculated by using the thermionic emission theory and indicated the Fermi level pinning effect occurring near the charge neutrality level of Si.This level is situated at an intermediate position between the mid-gap and the valence band edge of Si. 21,22) In contrast, the I −V curve for the W/Sb 2 Te 3 /n-Si heterojunction exhibited ohmic behavior, with the total resistance nearly matching the resistance of the Si substrate.Since it was not feasible to accurately estimate the electron barrier height form the I−V curve at RT, we investigated the temperature dependence of the I−V characteristics for the Sb 2 Te 3 /n-Si heterojunction within the range of 200−300 K.As a result, the electron barrier height was deduced to be 0.15 eV using a Richardson plot, 23,24) as shown in Fig. 3.By contrast, the I−V characteristics of the Sb 2 Te 3 /p-Si heterojunction exhibited clear rectification behavior, featuring a hole barrier height of 0.72 ± 0.01 eV and the ideality factor of 1.28 ± 0.15.Subsequently, subjecting the Sb 2 Te 3 /p-Si heterojunction to H 2 gas annealing at 400 °C for 10 min resulted in an improved ideality factor of 1.09 ± 0.02 and a hole barrier height of 0.77 eV ± 0.02 eV, as shown in Fig. 2(b).This improvement in the ideality factor can be attributed to the passivation of interfacial states on the Si surface by hydrogen and implies that the interaction between Sb 2 Te 3 and Si occurs through vdW force, as opposed to the formation of chemical bonds.It should be noted that, in a conventional metal/Si Schottky junction, chemical bonding or the formation of a silicide occurs between the metal and Si, thereby making characteristics insensitive to the hydrogen passivation effects.These electrical properties indicate that the Sb 2 Te 3 /p-Si heterojunction, with hydrogen passivation, exhibits good interface quality and behaves as a nearly ideal diode.Besides, such a high hole barrier height cannot be achieved in the conventional metal/p-Si junctions. 22)This unique feature confers distinct advantages in the application of Schottky diodes, manifesting in a significantly enhanced rectification ratio of forward bias to reverse bias current.It has been reported that there can be a low rectification ratio of the Sb 2 Te 3 /p-Si junction when the Si surface is left unpassivated by hydrogen. 19)In that case, the electrical properties of the Sb 2 Te 3 /Si heterojunction are influenced by the presence of interface states.][27][28][29][30] To evaluate the eWF, we also investigated the thickness (t) dependency of the SiO 2 film, as shown in Fig. 4(b).The flat-band voltage (V fb ) of Sb 2 Te 3 (t = 30 nm) was estimated to be 0.14 V from the intercept on the V fb axis.Accordingly, the eWF of the Sb 2 Te 3 film was obtained from the sum of the V fb , the Si electron affinity of 4.05 eV, and the difference between the conduction band edge and the Fermi level of 0.25 eV in the n-Si substrate with a dopant concentration of ∼5 × 10 15 cm −3 , resulting in the eWF = 4.44 eV.According to this result, band diagrams of Sb 2 Te 3 /n-Si and Sb 2 Te 3 /p-Si are depicted in Figs.5(a)−5(c).Assuming that Sb 2 Te 3 has the energy band gap (E g ) of 0.3 eV and Fermi level located at mid-gap, 31,32) the valence band offset (ΔE v ) is estimated to be 0.58 eV, and the conduction band offset (ΔE c ) is estimated to be 0.24 eV.It is noteworthy that the eWF exhibits a slight dependence on the thickness of the Sb 2 Te 3 on SiO 2 , as shown in Fig. 4(c).This dependency arises from the enhanced crystallinity of the Sb 2 Te 3 layer on the amorphous SiO 2 film with increasing thickness, indicating the necessity for a thicker Sb 2 Te 3 layer to investigate the intrinsic properties of Sb 2 Te 3 on SiO 2 .For I-V measurements, 20 nm Sb 2 Te 3 was directly deposited on Si substrates where superior  crystal quality could be obtained compared with the case on SiO 2 , leading to the reduction of eWF.Meanwhile, in Sb 2 Te 3 , the electron concentration is nearly equal to the hole concentration at RT due to the strong thermal excitation of carriers. 29,30)his phenomenon facilitates electron transport within the conduction band in Sb  036503-4 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd

Figure 4 (
a) shows the C−V characteristics of the capacitors at RT.The C-values in the positively biased accumulation region remained unchanged, irrespective to the thickness of Sb 2 Te 3 .Additionally, the C −V curves did not exhibit frequency dependence in the range of 10 3 −10 6 Hz, indicating that the Sb 2 Te 3 film works as a conductive electrode with a negligible parasitic capacitance.

Fig. 3 .
Fig. 3. (a) The temperature dependence of the I−V characteristics for the Sb 2 Te 3 /n-Si heterojunction within the range of 200−300 K. (b) Richardson plots of ln (I 0 /T 2 ) versus 1000/T for the Sb 2 Te 3 /n-Si heterojunction.Here, I o is the reverse saturation current, and T is the absolute temperature in Kelvin.Activation energy of 0.15 eV was obtained from the slope of the linear fit.

Fig. 4 .
Fig. 4. (a) The capacitance-voltage (C−V ) characteristics of the MOS capacitors (SiO 2 thickness = 25 nm) at RT with the Sb 2 Te 3 electrode at a frequency of 10 3 and 10 6 Hz.The inset exhibits the magnified figure of the C−V characteristics of the Sb 2 Te 3 electrode with a thickness (t) of 10, 20, and 30 nm.(b) The flat-band voltage V fb of the Sb 2 Te 3 electrode with t = 10, 20, and 30 nm as a function of the SiO 2 thickness.(c) The effective work function (eWF) of Sb 2 Te 3 on SiO 2 as a function of the Sb 2 Te 3 thickness, alongside the eWF of Sb 2 Te 3 directly on Si.

2
Te 3 .Namely, the ΔE c and ΔE v directly influences the electron and hole barrier heights, respectively, in the I−V measurements of the Sb 2 Te 3 /Si heterojunction.Note that, at the interface between W and Sb 2 Te 3 , the carrier readily tunnels through the very narrow Schottky barrier formed in Sb 2 Te 3 with a high carrier concentration, leading to a negligible impact of the W/Sb 2 Te 3 interface on the electrical properties of the whole W/Sb 2 Te 3 /Si heterojunctions.We then compared the Fermi level of the Sb 2 Te 3 /SiO 2 /Si capacitors with that of the Sb 2 Te 3 /Si junctions.Figures 5(d)−5(f) show band diagrams with ΔE c and ΔE v determined from I−V characteristics.The slight difference from Figs. 5(a)−5(c) could be attributed to differences in film quality of Sb 2 Te 3 ; the Sb 2 Te 3 on Si exhibits an improvement in crystalline ordering compared to that on SiO 2 , 20) as shown in Fig. 4(c).In conclusion, the I−V characteristics of Sb 2 Te 3 /Si indicated that the Sb 2 Te 3 film forms the high-quality heterojunction with Si.Sb 2 Te 3 /n-Si exhibited ohmic behavior, featuring a low-barrier heterojunction of 0.15 eV to n-Si, while Sb 2 Te 3 /p-Si exhibited rectification behavior, characterized by a high-barrier heterojunction of 0.72−0.77eV to p-Si.Furthermore, the C−V characteristics of the MOS capacitors with the Sb 2 Te 3 electrode provided insights into the band alignment of the Sb 2 Te 3 /Si heterojunctions, revealing ΔE c of 0.24 eV and ΔE v of 0.58 eV.Consequently, the Sb 2 Te 3 /n-Si heterojunction is expected to minimize contact resistance in high performance Si devices.Additionally, the Sb 2 Te 3 /p-Si heterojunction proves advantageous for high-quality Si diodes.

Fig. 5 .
Fig. 5. Band diagrams of (a)(d) the Si and Sb 2 Te 3 , (b)(e) the Sb 2 Te 3 /n-Si heterojunction, and (c)(f) the Sb 2 Te 3 /p-Si heterojunction at RT. (a)−(c) The work function j of Sb 2 Te 3 was determined by C−V characteristics, assuming an energy band gap E g of Sb 2 Te 3 to be 0.3 eV, and the Fermi level of Sb 2 Te 3 positioning at the mid-gap.(d)−(f) ΔE c and ΔE v were determined by temperature dependent I−V characteristics, showing a slightly smaller estimation of j of Sb 2 Te 3 than that estimated by C−V characteristics.