Optical and electrical properties of Al:WS₂ films prepared by atomic layer deposition and vulcanization

Abstract

Composition, structure, optical and electrical properties of Al:WS₂ (un-doped and Al-doped WS₂) films prepared by atomic layer deposition (ALD) and CS₂ vulcanization processing have been studied. Results show that Al:WS₂ films grow with a preferential c_⊥-orientation. The core-level binding energies (BEs) of W 4f, S 2p, O 1s and Al 2s decrease with increasing Al doping content, indicating that Al-doped WS₂ films have a p-type conductivity. Optical property analysis shows that the absorption coefficient (∼10⁷ m⁻¹) is comparable to that of WS₂ single crystals and that Al doping content can tune the optical band gap of the films. Hall measurements show that p-type conductive Al-doped WS₂ films can be obtained by Al doping. Hall mobility values for the un-doped WS₂ and 2.40% Al-doped WS₂ films are 1.63 × 10¹ and 9.71 cm² V⁻¹ s⁻¹, respectively. Comparing with un-doped WS₂ films, the comparable Hall mobility of Al-doped WS₂ films can be achieved by appropriate Al doping contents.

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1. Introduction

Tungsten disulfide (WS₂), as a member of the layered transition metal dichalcogenide (TMD) family, has been investigated extensively due to their use in applications such as new generation Field-Effect Transistors (FETs) and solar cells.^1–4 In fact, the performance of these applications depends on the optical and electrical properties of the channel material (for FETs) or absorbed layer (for solar cells). For example, WS₂ has a suitable band gap, which is reasonable for the replacement of silicon in complementary metal oxide semiconductor (CMOS) digital logic devices;⁵ moreover, the high absorption coefficient and tunable band gap are significantly important for the utilization of solar energy.^2,6,7 Therefore, it is worth studying the optical and electrical properties of WS₂ materials. In general, a p-type semiconductor is an indispensable requirement for p–n junction solar cells and CMOS integrated circuits. Thus, it is necessary to investigate the preparation and properties of p-type semiconductors. Doping can tune the electrical and optical properties of the materials and achieve the p-type conductive characteristics,⁸ and then the doped materials can be applied to optical and electrical applications. For the abovementioned analysis, it is meaningful to study the optical and electrical properties of p-type doped WS₂ films. In view of the lower valence electron number, similar electronegativity and ionic radius (compared with W atoms), Al can be more easily incorporated into the crystal lattice of WS₂ (ref. 9) to achieve p-type doping.

Researchers have prepared WS₂ films by a two-step method^10–13 (the first step: preparation of W or WO₃ films via sputtering or ALD; the second step: synthesis of WS₂ films via S powder or H₂S vulcanization). For the first step, the films deposited by ALD have better uniformity and a controlled atomic layer thickness, which is preponderant in the applications. Therefore, in this study, ALD has been chosen to prepare Al:WO_x (un-doped and Al-doped WO_x) films. For the second step, S powder has a lower reducibility than H₂S gas and H₂S gas is highly toxic and expensive. Therefore, a new deoxidizer needs to be explored to replace S powder and H₂S gas. CS₂, on the other hand, has a low toxicity, low cost and maintains a liquid state under normal temperatures, allowing it to be easily controlled. Therefore, Al:WS₂ films were synthesized by ALD and CS₂ vulcanization for the first time in this study. Effects of the Al doping content on the composition, structure, optical and electric properties of the Al:WS₂ films were investigated systematically.

2. Experimental

Al:WO_x thin films were deposited on quartz substrates at 150 °C using the KMP-200A plasma enhanced-ALD system. Before deposition, the quartz substrates were washed ultrasonically with deionized water, acetone and then ethanol in turn. Aluminium methide (Al(CH₃)₃), bis(tert-butylimido)-bis(dimethylamido)tungsten ((^tBuN)₂(Me₂N)₂W) and O plasma were used as the precursors for Al, W and O, respectively. High purity Ar (99.99%) was used as the purging gas. The Al and W precursor vessels were maintained at 0 and 100 °C, respectively. The pulse times of Al(CH₃)₃, ((^tBuN)₂(Me₂N)₂W) and O plasma vapor were 0.02, 1 and 15 s, respectively. The growth rate of Al₂O₃ and WO_x films were about 0.120 nm and 0.108 nm per cycle, respectively. To achieve Al doping, one Al(CH₃)₃ + O plasma cycle was introduced into different number cycles of the ((^tBuN)₂(Me₂N)₂W) + O plasma cycle, which can be shown in the following expression: [a × (W(N^tBu)₂(NMe₂)₂ + O plasma) + (Al(CH₃)₃ + O plasma)] × b. Therefore, Al doping content can be tuned by the a value in the expression. After ALD deposition, Al:WS₂ thin films were synthesized in the tube furnace through CS₂ vulcanization. Fig. 1(a) shows the schematic of the vulcanization of Al:WO_x films. At first, the tube furnace was pumped down to ∼10⁻² Pa and then flushed with pure Ar to a normal pressure; this process was repeated twice. Second, the samples were heated to 700 °C (10 °C min⁻¹) and vulcanized for 30 min under normal pressure. During the vulcanization, a mixture of 50 SCCM Ar carrier gas with CS₂ was introduced into the quartz tube for the reaction to occur.

Fig. 1 Schematic of the sulfurization of Al:WO_x films (a) and XRD patterns of Al:WS₂ thin films at different Al doping contents (b).

The thicknesses (30 nm) of the Al:WS₂ thin films were measured by a UVISEL ER spectroscopic phase modulated spectroscopic ellipsometry system. X-ray diffraction (XRD) was conducted for structure analysis of the films with monochromatic Cu Kα radiation (40 kV, 45 mA, λ = 1.54178 Å) on an X'Pert Pro MPD X-ray diffractometer. X-ray photoelectron spectroscopy (XPS) measurements were performed to determine the chemical composition and states analysis using a monochromatic Al Kα (1486.6 eV) X-ray source. UV-3150 Shimadzu ultraviolet-visible-near infrared scanning spectrophotometer was used to measure the optical transmittance and reflectance of the films in the wavelength range of 400–1500 nm. Hall measurements of the Al:WS₂ thin films were performed on a HMS-5000 system by utilizing the van der Pauw method.

3. Results and discussion

Fig. 1(b) shows the XRD patterns of Al:WS₂ thin films prepared at different Al doping contents. The XRD pattern comprises of only one peak at a 2θ value of ∼14.32°, which can be indexed to the (002) plane of 2H-WS₂ (JCPDS card 08-0237). Al:WS₂ thin films exhibit a (002) preferential orientation (c-axis perpendicular to the substrate), which is considered a type II texture by Sadale et al.¹⁴ As shown in Fig. 1, the (002) peak intensity of the un-doped WS₂ films is the highest, and the intensity decreases after Al doping, indicating that Al doping leads to a reduction in the crystallinity. The same phenomenon can be found in Al-doped WS₂ films investigated by Xu et al.¹⁵ and Co-doped WS₂ nanorods prepared by Wang et al.¹⁶ Although the (002) peak intensity for the 8.16% Al-doped WS₂ films increased slightly compared with 5.19% Al, it is worth noting that the intensity of the (002) peak appears to have a trend of reduction as a whole with the increment of the Al doping content. The slight augmentation in the (002) peak intensity may be due to a moderate quantity of Al atoms existing at interstitial sites or interlaminating, together with W atoms sharing S atoms.^17,18 From the abovementioned structure analysis, the crystallinity of the Al:WS₂ film meliorates with the reduction of the Al doping content, showing that the amount of Al doping content needs to be controlled.

The chemical composition and chemical states of the Al:WS₂ thin films were studied by XPS analysis. Table 1 shows the summary of valence states, peak position and relative content of the Al:WS₂ thin films. The total concentration of W and S elements decreases with the increase of the Al doping content, while the overall O and Al elements increase. As the Al doping content increases, the core-level BEs of the W 4f, S 2p, O 1s and Al 2s decrease, which is due to charge transfer and a change in the environmental charge density.¹⁹ Chen et al.²⁰ studied doping MoS₂ and found that the downshift of the BEs is related to the reduction in energy difference between the Fermi level and the valence band edge. Yang et al.²¹ have investigated Cl-doped WS₂ and confirmed an n-type doping based on the increase of the W 4f_7/2 and S 2p_3/2 core-level BEs. Therefore, the reduction of the W 4f, S 2p, O 1s and Al 2s core-level BEs illustrates that the Fermi level is close to the valence band and that p-type doping is obtained after Al doping. Fig. 2 shows the XPS spectra for (a) W 4f, (b) S 2p and (c) O 1s for the un-doped WS₂ films. As shown in Fig. 2(a), the W 4f peak can be fit to two spin–orbit doublets belonging to W⁴⁺–S and W⁶⁺–O. W 4f_7/2 BEs for W⁴⁺–S and W⁶⁺–O are at 33.19 eV and 36.21 eV, which is in agreement with the reported W 4f_7/2 BEs of WS₂ (ref. 22) and WO₃,²³ respectively. As shown in Fig. 2(b) and (c), the S 2p and O 1s both correspond to only one chemical state: S²⁻–W and O²⁻–W. BEs of S 2p_3/2 (at 162.84 eV (ref. 22)) and O 1s (at 530.90 eV (ref. 24)) further illustrate the chemical states of the W cations. When 2.40% Al is introduced into the WS₂ films, as shown in Fig. 3(a)–(d), the S 2p and O 1s both represent two chemical states: the S 2p peak is deconvoluted into two doublets for S²⁻–W and S²⁻–Al and the O 1s peak is deconvoluted into two peaks for O²⁻–W and O²⁻–Al. The BEs of S 2p_3/2 and O 1s for these four chemical states are at 162.67 eV, 162.90 eV, 530.72 eV and 531.88 eV.^13,25–27 The Al 2s peak is chosen here to better analyze the chemical states of Al because it is hard to distinguish between the Al 2p and W 5s peaks (BEs of Al 2p and W 5s are both at ∼75 eV (ref. 28 and 29)). Chemical states (assigned to Al³⁺–S and Al³⁺–O) of Al can be determined via peak separation of the O 1s and S 2p peaks. Part of the Al³⁺ ions replace W⁴⁺ ions bonded to S and the other part of the Al³⁺ ions combine with O. BEs of Al 2s are consistent with the reported results.^30,31 With further increases in the Al doping content, chemical states of the W 4f, S 2p, O 1s and Al 2s peaks are consistent with those of the 2.40% Al-doped WS₂ films, BEs of the four peaks agree well with the reported literature.^9,30–34

Table 1 Summary of valence states, peak position and relative content (at%) of W, S, O and Al elements in Al:WS₂ thin films^a

		W		S		O		Al	W 5p3/2
		W 4f7/2		S 2p3/2		O 1s		Al 2s
		W^4+–S	W^6+–O	S^2−–W	S^2−–Al	O^2−–W	O^2−–Al
a Notation: W^4+–S represents two kinds of meanings, namely, W^4+ bonding with S and the content of W^4+ bonded with S, which is the same with W^6+–O, S^2−–W, O^2−–Al, and Al^3+–S. W represents the content of total W element (including W^4+–S and W^6+–O), which is the same with Se, O and Al.
Un-doped	BE (eV)	33.19	36.21	162.82		530.87			38.97
	(at%)	32.06	0.95	64.14		2.85
2.40% Al	BE (eV)	33.05	36.04	162.67	162.90	530.72	531.88	119.64	38.82
	(at%)	30.1	0.95	60.10	2.60	2.84	1.01	2.4
3.54% Al	BE (eV)	32.92	35.90	162.55	162.77	530.59	531.77	119.51	38.66
	(at%)	29.15	0.98	58.08	4.04	2.95	1.26	3.54
5.19% Al	BE (eV)	32.73	35.71	162.36	162.57	530.40	531.58	119.31	38.48
	(at%)	27.81	0.97	55.33	6.21	2.92	1.57	5.19
8.16% Al	BE (eV)	32.48	35.46	162.13	162.32	530.14	531.32	119.04	38.24
	(at%)	25.51	0.97	50.23	9.34	2.90	2.89	8.16

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Fig. 2 XPS spectra of un-doped WS₂ films: (a) W 4f; (b) S 2p; and (c) O 1s.

Fig. 3 XPS spectra of Al:WS₂ films with a 2.40% Al doping content: (a) W 4f; (b) S 2p; (c) O 1s; (d) Al 2s.

Fig. 4(a)–(c) shows the optical transmittance (T) (inset: reflectance (R)), absorption coefficient (α) and optical band gap (E_g) of the Al:WS₂ thin films. As shown in Fig. 4(a) and (b), the average optical transmittance of the films increases with increasing Al doping content, while the absorption coefficient decreases. The absorption coefficient (α) is calculated from the optical transmittance and reflectance based on the following expression:^35,36

where d is the thickness. As observed, the values of α are at ∼10⁷ m⁻¹, which is comparable to the values of WS₂ single crystals³⁷ and higher than WS₂ films prepared by sputtering + H₂S vulcanization.¹³ The peaks at ∼1.94 eV (A point) and ∼2.35 eV (B point) correspond to the excitonic nature of the Al:WS₂ films.³⁷ Yen et al.³⁸ ascribed A and B excitons to the smallest direct transitions at the K point of the Brillouin zone, with A and B excitons corresponding to K₄ → K₅ and K₁ → K₅ transitions, respectively. The optical band gap E_g is given by the follow relation:³⁶

αhν = A(hν − E_g)ⁿ (2)

where A is the constant and hν is the incident photo energy, n is 1/2 and 2 for a direction allowed transition and an indirect allowed transition, respectively. As previously reported,^36,39,40 Al:WS₂ films are indirect band gap semiconductors. Fig. 4(c) shows the plot of (αhν) ^1/2 versus hν of the Al:WS₂ films. The optical band gaps of the Al:WS₂ films are higher than the indirect band gap (1.30 eV (ref. 41)) of WS₂ films owing to the existence of the tungsten oxidation state.⁴² As the Al doping content increases to 3.54%, the optical band gap decreases from 1.49 eV (un-doped) to 1.36 eV (3.54% Al). The reduction in the optical band gap is attributed to the defect energy levels and localized states introduced in the band structure after Al doping.⁴³ As Al doping content further increases, the optical band gap increases to 1.47 eV (8.16% Al), which is due to the increase in the Al and O concentrations. This increment enhances the Al–O bond interrelation and leads to the bottom of the conduction band moving up, resulting in the augmentation of the optical band gap. From the abovementioned analysis, the incorporation of the Al element could tune the optical properties of the Al:WS₂ films.

Fig. 4 Optical transmittance spectra (a), absorption coefficient (b) and optical band gaps (c) of the Al:WS₂ films as a function of Al doping content. The inset of (a) shows the reflectance of the Al:WS₂ films as a function of wavelength.

Fig. 5(a)–(d) shows the carrier concentration (n), the room temperature resistivity (ρ), the Hall mobility (μ_H) and the Hall coefficients (R_H) of Al:WS₂ films at different Al doping contents. As observed, the negative R_H of the un-doped WS₂ films shows stable n-type conduction characteristic. After Al doping, the values of R_H becomes positive, suggesting that Al-doped WS₂ films have p-type conductivity. As shown in Fig. 5(a) and (b), with the increase in the Al doping content, n values increase first and then decrease, and ρ values increase. The augment of the ρ values is affected by the incremental content of the Al₂O₃ insulating phase (shown in Table 1, O²⁻–Al) in the films. Shown in Fig. 5(c), the μ_H values of the un-doped WS₂ and 2.40% Al:WS₂ films are 1.63 × 10¹ and 9.71 cm² V⁻¹ s⁻¹, respectively, which is comparable to that of the WS₂ films prepared by van der Waals rheotaxy (9–15 cm² V⁻¹ s⁻¹).⁴¹ With Al doping content increasing to 8.16%, the value of μ_H decreases to 6.18 × 10⁻¹ cm² V⁻¹ s⁻¹. The variation of μ_H depends on the scattering mechanisms present in the Al:WS₂ films. These mechanisms include lattice scattering, ionized impurity scattering, neutral impurity scattering, and grain boundary scattering.⁴⁴ According to the analysis by Chen et al.,⁴⁴ the reduction of the μ_H in our study is primarily because of the ionized impurity scattering. The acceptor impurity with a negative charge is produced after Al doping, leading to the formation of a Coulomb potential field around the acceptor impurity, which destroys the periodic potential field near the impurity and then scatters the carriers. Thus, the introduced acceptor impurity increases and become the ionization scattering center with the increase in the Al doping content, finally resulting in the reduction of the μ_H. Consequently, it can be concluded that a p-type conduction characteristic can be obtained by Al doping and the appropriate Al doping content can achieve a comparable μ_H to the un-doped WS₂ films.

Fig. 5 (a) Carrier concentration (n), (b) resistivity (ρ), (c) Hall mobility (μ_H) and (d) Hall coefficients (R_H) of the Al:WS₂ films with different Al doping contents.

4. Conclusion

In summary, Al:WS₂ films were synthesized by ALD and CS₂ vulcanization processing. Structure analysis shows that Al:WS₂ films grow along the (002) direction, and the (002) peak intensity and crystalline quality increases with the reduction of the Al doping content. XPS analysis shows that the core-level binding energies (BEs) of the W 4f, S 2p, O 1s and Al 2s decrease with increasing Al doping concentration, indicating that the Fermi level is close to the valence band and p-type doping is obtained after Al doping. Optical and electrical property analyses show that with the increase of Al doping content, the resistivity increases gradually, the Hall mobility and the absorption coefficients decrease, the optical band gap decreases first and then increases, and the carrier concentration increases first and then decreases. Moreover, a p-type conduction characteristic can be obtained by Al doping, and the comparable Hall mobility of the Al-doped WS₂ films can be achieved by an appropriate Al doping content.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (Contract No. 61376091), the Fundamental Research Funds for the Central Universities (Contract No. 3102014JCQ01033), the Aeronautical Science Foundation of China (Contract No. 2014ZF53070) and the Research Fund of the State Key Laboratory of Solidification Processing (NWPU) of China (Contract No. 155-QP-2016).

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