Gate tunable spatial accumulation of valley-spin in chemical vapor deposition grown 40°-twisted bilayer WS<sub>2</sub>

Siwen Zhao; Gonglei Shao; Zheng Vitto Han; Song Liu; Tongyao Zhang

doi:10.1088/1674-4926/44/1/012001

J. Semicond. > 2023, Volume 44 > Issue 1 > 012001

ARTICLES

Gate tunable spatial accumulation of valley-spin in chemical vapor deposition grown 40°-twisted bilayer WS₂

Siwen Zhao¹, Gonglei Shao², Zheng Vitto Han^{3, 4}, Song Liu^2, and Tongyao Zhang^{3, 4,}

+ Author Affiliations

1. Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110010, China
2. Institute of Chemical Biology and Nanomedicine, State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China
3. State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Opto-Electronics, Shanxi University, Taiyuan 030006, China
4. Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China

Author Bio:

Siwen Zhao received his BS degree from Shandong University and then finished his Ph.D. degree from the Department of Physics in the University of Science and Technology of China. He held a position of postdoctoral fellow in Shenzhen University from 2019 to 2022. Currently, he is working as a Research Associate in the Institute of metal research, Chinese Academy of Sciences. His recent research interests focus on the electronic transport properties and potential applications of nanometer-scale devices based on novel 2D materials and their heterostructures.

Song Liu is a full Professor at the College of Chemistry and Chemical Engineering, Hunan University. He received his BS degree from Nankai University and then finished his Ph.D. degree from Peking University. After that he worked at Case Western Reserve University and National University of Singapore as postdoctoral fellow. His research interests focus on the low dimensional materials and functional devices, as well as the applications in energy and biosensors.

Tongyao Zhang is a lecturer at the Institute of opto-electronics, Shanxi University. He received his BS degree and Ph.D. degree from Shanxi University. He mainly focuses on low-temperature microscopic spectroscopy of atomically thin van der Waals heterostructures by optical probe and optical manipulation.

Corresponding author: Song Liu, liusong@hnu.edu.cn; Tongyao Zhang, tongyao_zhang@sxu.edu.cn

DOI: 10.1088/1674-4926/44/1/012001

Abstract: The emerging two-dimensional materials, particularly transition metal dichalcogenides (TMDs), are known to exhibit valley degree of freedom with long valley lifetime, which hold great promises in the implementation of valleytronic devices. Especially, light–valley interactions have attracted attentions in these systems, as the electrical generation of valley magnetization can be readily achieved — a rather different route toward magnetoelectric (ME) effect as compared to that from conventional electron spins. However, so far, the moiré patterns constructed with twisted bilayer TMDs remain largely unexplored in regard of their valley spin polarizations, even though the symmetry might be distinct from the AB stacked bilayer TMDs. Here, we study the valley Hall effect (VHE) in 40°-twisted chemical vapor deposition (CVD) grown WS₂ moiré transistors, using optical Kerr rotation measurements at 20 K. We observe a clear gate tunable spatial distribution of the valley carrier imbalance induced by the VHE when a current is exerted in the system.

Key words: transition metal dichalcogenides, valleytronic devices, light–valley interactions, valley Hall effect

1. Introduction

Zinc oxide is a very important material for electronic and optoelectronic devices because of its excellent properties of direct and wide band gap and large exciton binding energy^{[1, 2]}. The greatest challenge for these applications,however,remains the fabrication of reliable and stable p-type ZnO with low resistivity and high carrier concentration^[3]. This p-type doping issue is related to the formation and compensation effects of native donor defects such as O vacancies and Zn interstitials,low solubility of p-dopants and higher activation energy of acceptors in the band gap of ZnO^{[4, 5, 6]}. It is reported in the literature that even nitrogen and phosphorous are the most favorable p-type dopants in ZnO,but stable p-type conductivity is hard to achieve due to the low solubility^[7]. The solubility limit for acceptor impurities in ZnO,as well as other wide-gap materials suffering from doping asymmetry,can be enhanced above the thermodynamic limit by employing ``non-equilibrium'' conditions. One way to improve the dopant solubility is through adjusting dopant chemical potential $\mu$ A. The chemical potential of an acceptor dopant in the ZnO matrix determines the dopant solubility^[8]. Therefore,raising this potential would increase the solubility of the dopant coupled with avoidance of precipitate formation. High temperature annealing is a very efficient way to facilitate the enhancement of P concentration. Furthermore,high temperature annealing can activate the P atoms in ZnO,suppress the formation of P $_{\rm O}$ defects and reduce the density of oxygen vacancies^[9]. Therefore,a high temperature annealing study would be very helpful to grow stable p-type ZnO.

In this paper,we have achieved high solubility of phosphorous in bulk ZnO by using high temperature annealing conditions. The ZnO pellets were annealed from 500 to 1000 ℃ in a programmable furnace; it was found that the maximum concentration of P occurred in a sample annealed at 800 ℃. The results were explained with the help of XRD,PL and Hall measurements.

2. Experimental

ZnO pellets were synthesized by the conventional sintering technique in an atmospheric furnace using a zinc oxide powder (99.99%). The ZnO powders were first milled in an agate mortar and heated in air at an annealing temperature of 300 ℃ for 2 h to evaporate the water and remove the organic residuals. The obtained powders were then pressed into pellet discs (of about 1 mm in thickness and 8 mm in diameter). A drop of phosphors dopant P430 was sprayed on the surface of pellets and then subjected to the annealing process. The pellets annealed at different temperatures from 500 to 1000 ℃,keeping a step of 100 ℃ using a programmable furnace for one hour.

The characterization of annealed samples was done by using following equipments; SEM/EDX (3000 Hitachi),PL/Raman (Photon Systems) having laser wavelength 248 nm,Hall effect by Ecopia 3000 and XRD (miniflex Rigaku). All the measurements were performed at room temperature.

3. Results and discussion

3.1 EDX measurements

shows the EDX results of P-doped ZnO annealed at different temperatures from 500 to 1000 ℃. The figure shows that the concentration of phosphorous increases with annealing temperature up to an annealing temperature of 800 ℃ and again decreases for samples annealed at higher temperatures. Moreover,the concentration of oxygen content in the samples also increases with annealing temperature. The O-rich growth conditions suppress the formation of P $_{\rm O}$ acceptor defects and instead increase the probability of generating Zn vacancies and O-interstitials due to the Zn deficiency conditions. Among them,Zn vacancy-related defects result in the rapid incorporation of P atoms in the Zn sites,and the ultimate result is the formation of a P $_{\rm Zn}$ -2V $_{\rm Zn}$ acceptor complex^[10]. The formation mechanism of this acceptor complex has already been reported by many authors^{[11, 12]}. To verify the p-type conductivity of annealed samples,we have performed Seebeck measurements. The results suggested that samples annealed at 800 and 900 ℃ showed p-type conductivity and the rest all showed n-type conductivity. This result confirmed that the maximum diffusion of P atoms into ZnO pellets occurred at an annealing temperature of 800 ℃.

Figure 1. EDX measurements of bulk ZnO annealed at different temperatures from 500 to 1000 ℃.

DownLoad: Full-Size Img PowerPoint

3.2 XRD measurements

Typical XRD patterns of the P-doped ZnO pellets sintered at different temperatures (500 to 1000 ℃ with a step of 100 ℃) for one hour demonstrated `2 $\theta$ values' of 8 diffraction peaks corresponding to the ZnO (100),(002),(101),(102),(110),(103),(200) and (112) planes,respectively. A comparison with the JCPD 36-1451 Card confirmed the formation of hexagonal zinc oxide^{[13, 14]}. For clarity,we have displayed only the dominant diffraction peaks corresponding to the (002) plane,in . We clearly see that the 2 $\theta$ value of the major peak of the representative ZnO samples varies with the sintering temperature. The 2 $\theta$ value of the peak increases and reaches 34.6 $^\circ$ at the annealing temperature of 800 ℃ shown in the inset of . According to the size consideration of P,an increase in lattice constant is expected when a P atom occupies an O site (smaller in size than P); otherwise,the lattice constant should decrease when a P atom occupies a Zn site (larger in size than P)^[15]. Practically,the (002) plane of the ZnO shifts towards a higher 2 $\theta$ value with the sintering temperatures up to 800 ℃ and thus yields a smaller lattice constant. Henceforth,we will refer the filling of Zn site with P as an A-type shift. Accordingly,the information from the literature suggest that a P atom while occupying a Zn site may generate two Zn vacancies and hence produces a shallow acceptor P $_{\rm Zn}$ -2V $_{\rm Zn}$ complex above the valence band maximum.

Figure 2. (Color online) X-ray diffraction pattern of P-doped bulk ZnO annealed from 500 to 1000 ℃. Inset demonstrated the effect of annealing temperature on the peak position of ZnO (002) plane.

DownLoad: Full-Size Img PowerPoint

3.3 PL spectroscopy

The room temperature PL measurements were carried out to examine the effect of annealing on the optical properties of the P-doped ZnO pellets. Figure3 compares the photoluminescence spectra of doped samples measured at room temperature. All samples consist of a band to band emission at 3.29 eV,but samples annealed at 800,900 and 1000 ℃ have an additional peak at 3.2 eV^[16]. This additional peak is related with the donor acceptor pair,where the acceptor is the P atom.

Figure 3. (Color online) Photoluminescence spectra of P-doped ZnO powder annealed at 500-1100 ℃.

DownLoad: Full-Size Img PowerPoint

3.4 Hall measurements

The Hall measurements were performed to calculate the carrier concentration of P-doped samples and shown in . The graph shows that the hole concentration is maximum for a sample annealed at 800 ℃. The observed result can be explained as follows: as the annealing temperature increases the P doping increases,which formed an acceptor complex level with V $_{\rm Zn}$ ,therefore the carrier concentration increases. All other samples show the n-type conductivity (not shown here).

Figure 4. Annealing temperature versus carrier concentration graph.

DownLoad: Full-Size Img PowerPoint

4. Conclusion

In conclusion,we have successfully enhanced the phosphorous diffusion in ZnO pellets to achieve p-type conductivity. ZnO pellets were doped with P using high temperature annealing conditions. The maximum diffusion of P atoms occurred at the annealing temperature of 800 ℃,confirmed by EDX measurements. The shifting of the (002) ZnO plane towards higher 2 $\theta$ angles in XRD and emerging of donor acceptor pair in PL spectra at the annealing temperature of 800~℃ strongly suggested that maximum diffusion of P atoms occurred at the annealing temperature of 800 ℃.

Acknowledgements

Authors are thankful to the Higher Education Commission (HEC) of Pakistan for the financial assistance under project # IPFP/HRD/HEC/2014/2016.

References

[1]	Zhao S W, Li X X, Dong B J, et al. Valley manipulation in monolayer transition metal dichalcogenides and their hybrid systems: Status and challenges. Rep Prog Phys, 2021, 84, 026401 doi: 10.1088/1361-6633/abdb98
[2]	Mak K F, Xiao D, Shan J. Light–valley interactions in 2D semiconductors. Nat Photonics, 2018, 12, 451 doi: 10.1038/s41566-018-0204-6
[3]	Seyler K L, Zhong D, Huang B, et al. Valley manipulation by optically tuning the magnetic proximity effect in WSe₂/CrI₃ heterostructures. Nano Lett, 2018, 18, 3823 doi: 10.1021/acs.nanolett.8b01105
[4]	Ciarrocchi A, Unuchek D, Avsar A, et al. Polarization switching and electrical control of interlayer excitons in two-dimensional van der Waals heterostructures. Nat Photonics, 2019, 13, 131 doi: 10.1038/s41566-018-0325-y
[5]	Li L F, Shao L, Liu X W, et al. Room-temperature valleytronic transistor. Nat Nanotechnol, 2020, 15, 743 doi: 10.1038/s41565-020-0727-0
[6]	Lee J, Mak K F, Shan J. Electrical control of the valley Hall effect in bilayer MoS₂ transistors. Nat Nanotechnol, 2016, 11, 421 doi: 10.1038/nnano.2015.337
[7]	Wu Z F, Zhou B T, Cai X B, et al. Intrinsic valley Hall transport in atomically thin MoS₂. Nat Commun, 2019, 10, 611 doi: 10.1038/s41467-019-08629-9
[8]	Barré E, Incorvia J A C, Kim S H, et al. Spatial separation of carrier spin by the valley Hall effect in monolayer WSe₂ transistors. Nano Lett, 2019, 19, 770 doi: 10.1021/acs.nanolett.8b03838
[9]	Lee J, Wang Z F, Xie H C, et al. Valley magnetoelectricity in single-layer MoS₂. Nat Mater, 2017, 16, 887 doi: 10.1038/nmat4931
[10]	Vitale S A, Nezich D, Varghese J O, et al. Valleytronics: Valleytronics: Opportunities, challenges, and paths forward (Small 38/2018). Small, 2018, 14, 1870172 doi: 10.1002/smll.201870172
[11]	Chen X Z, Shi S Y, Shi G Y, et al. Observation of the antiferromagnetic spin Hall effect. Nat Mater, 2021, 20, 800 doi: 10.1038/s41563-021-00946-z
[12]	Jungwirth T, Wunderlich J, Olejník K. Spin Hall effect devices. Nat Mater, 2012, 11, 382 doi: 10.1038/nmat3279
[13]	Wang L, Shih E M, Ghiotto A, et al. Correlated electronic phases in twisted bilayer transition metal dichalcogenides. Nat Mater, 2020, 19, 861 doi: 10.1038/s41563-020-0708-6
[14]	Li E, Hu J X, Feng X M, et al. Lattice reconstruction induced multiple ultra-flat bands in twisted bilayer WSe₂. Nat Commun, 2021, 12, 5601 doi: 10.1038/s41467-021-25924-6
[15]	Pacchioni G. Valleytronics with a twist. Nat Rev Mater, 2020, 5, 480 doi: 10.1038/s41578-020-0220-2
[16]	Shao G L, Xue X X, Liu X, et al. Twist angle-dependent optical responses in controllably grown WS₂ vertical homojunctions. Chem Mater, 2020, 32, 9721 doi: 10.1021/acs.chemmater.0c03413
[17]	Cao T, Wang G, Han W P, et al. Valley-selective circular dichroism of monolayer molybdenum disulphide. Nat Commun, 2012, 3, 887 doi: 10.1038/ncomms1882
[18]	Zhang T Y, Zhao S W, Wang A R, et al. Electrically and magnetically tunable valley polarization in monolayer MoSe₂ proximitized by a 2D ferromagnetic semiconductor. Adv Funct Mater, 2022, 32, 2204779 doi: 10.1002/adfm.202204779
[19]	Li X T, Liu Z D, Liu Y H, et al. All-electrical control and temperature dependence of the spin and valley Hall effect in monolayer WSe₂ transistors. ACS Appl Electron Mater, 2022, 4, 3930 doi: 10.1021/acsaelm.2c00599
[20]	Plechinger G, Nagler P, Kraus J, et al. Identification of excitons, trions and biexcitons in single-layer WS₂. Phys Status Solidi RRL, 2015, 9, 457 doi: 10.1002/pssr.201510224
[21]	Gong Y Y, Carozo V, Li H Y, et al. High flex cycle testing of CVD monolayer WS₂ TFTs on thin flexible polyimide. 2D Mater, 2016, 3, 021008 doi: 10.1088/2053-1583/3/2/021008
[22]	Wu K, Zhong H X, Guo Q B, et al. Revealing the competition between defect-trapped exciton and band-edge exciton photoluminescence in monolayer hexagonal WS₂. Adv Opt Mater, 2022, 10, 2101971 doi: 10.1002/adom.202101971
[23]	Hu Z L, Avila J, Wang X Y, et al. The role of oxygen atoms on excitons at the edges of monolayer WS₂. Nano Lett, 2019, 19, 4641 doi: 10.1021/acs.nanolett.9b01670
[24]	Kim M S, Yun S J, Lee Y J, et al. Biexciton emission from edges and grain boundaries of triangular WS₂ monolayers. ACS Nano, 2016, 10, 2399 doi: 10.1021/acsnano.5b07214
[25]	Chow P K, Jacobs-Gedrim R B, Gao J, et al. Defect-induced photoluminescence in monolayer semiconducting transition metal dichalcogenides. ACS Nano, 2015, 9, 1520 doi: 10.1021/nn5073495

Fig. 1. (Color online) (a) Twisted bilayer WS₂ crystal structure and schematic of the band extrema at the K⁺ and K⁻ points in monolayer WS₂. The figure shows the conduction-band and valence-band spin splitting and the allowed optical transitions for circularly polarized light. (b) Schematic of the twisted bilayer WS₂ (t-WS₂)/h-BN heterojunction device. Optical microscope images of (c) twisted bilayer WS₂ transistor on SiO₂/Si, (d) t-WS₂/h-BN heterostructure, and (e) the final device after lithography patterning. The scale bar is 5 μm.

DownLoad: Full-Size Img PowerPoint

Fig. 2. (Color online) (a) Source–drain current as a function of back-gate voltage V_g of the device at 20 K for V_ds =1 V. (b) PL spectra of the heterostructure at 20 K. In the PL plot, the thick green solid line indicates the measured data, and the red solid line shows the Gaussian fitting result. The four Gaussian components are attributed to the neutral excitons (X₀), trions (X^-), defect-trapped localized exciton (LX₁) or biexciton (XX), and defect-trapped localized exciton (LX₂).

DownLoad: Full-Size Img PowerPoint

Fig. 3. (Color online) Spatial map of the Kerr rotation angle under (a) V_g = –80 V, (b) V_g = 0 V, and (c) V_g = 20 V. Signals from metal electrodes are not shown as the polarization of reflected light from electrodes is destructed.

DownLoad: Full-Size Img PowerPoint

Fig. 4. (Color online) Linecuts of the Kerr rotation map under different back gates. Original data and fits are open markers and solid lines, respectively, which are shifted vertically for clarity.

DownLoad: Full-Size Img PowerPoint

[1]	Zhao S W, Li X X, Dong B J, et al. Valley manipulation in monolayer transition metal dichalcogenides and their hybrid systems: Status and challenges. Rep Prog Phys, 2021, 84, 026401 doi: 10.1088/1361-6633/abdb98
[2]	Mak K F, Xiao D, Shan J. Light–valley interactions in 2D semiconductors. Nat Photonics, 2018, 12, 451 doi: 10.1038/s41566-018-0204-6
[3]	Seyler K L, Zhong D, Huang B, et al. Valley manipulation by optically tuning the magnetic proximity effect in WSe₂/CrI₃ heterostructures. Nano Lett, 2018, 18, 3823 doi: 10.1021/acs.nanolett.8b01105
[4]	Ciarrocchi A, Unuchek D, Avsar A, et al. Polarization switching and electrical control of interlayer excitons in two-dimensional van der Waals heterostructures. Nat Photonics, 2019, 13, 131 doi: 10.1038/s41566-018-0325-y
[5]	Li L F, Shao L, Liu X W, et al. Room-temperature valleytronic transistor. Nat Nanotechnol, 2020, 15, 743 doi: 10.1038/s41565-020-0727-0
[6]	Lee J, Mak K F, Shan J. Electrical control of the valley Hall effect in bilayer MoS₂ transistors. Nat Nanotechnol, 2016, 11, 421 doi: 10.1038/nnano.2015.337
[7]	Wu Z F, Zhou B T, Cai X B, et al. Intrinsic valley Hall transport in atomically thin MoS₂. Nat Commun, 2019, 10, 611 doi: 10.1038/s41467-019-08629-9
[8]	Barré E, Incorvia J A C, Kim S H, et al. Spatial separation of carrier spin by the valley Hall effect in monolayer WSe₂ transistors. Nano Lett, 2019, 19, 770 doi: 10.1021/acs.nanolett.8b03838
[9]	Lee J, Wang Z F, Xie H C, et al. Valley magnetoelectricity in single-layer MoS₂. Nat Mater, 2017, 16, 887 doi: 10.1038/nmat4931
[10]	Vitale S A, Nezich D, Varghese J O, et al. Valleytronics: Valleytronics: Opportunities, challenges, and paths forward (Small 38/2018). Small, 2018, 14, 1870172 doi: 10.1002/smll.201870172
[11]	Chen X Z, Shi S Y, Shi G Y, et al. Observation of the antiferromagnetic spin Hall effect. Nat Mater, 2021, 20, 800 doi: 10.1038/s41563-021-00946-z
[12]	Jungwirth T, Wunderlich J, Olejník K. Spin Hall effect devices. Nat Mater, 2012, 11, 382 doi: 10.1038/nmat3279
[13]	Wang L, Shih E M, Ghiotto A, et al. Correlated electronic phases in twisted bilayer transition metal dichalcogenides. Nat Mater, 2020, 19, 861 doi: 10.1038/s41563-020-0708-6
[14]	Li E, Hu J X, Feng X M, et al. Lattice reconstruction induced multiple ultra-flat bands in twisted bilayer WSe₂. Nat Commun, 2021, 12, 5601 doi: 10.1038/s41467-021-25924-6
[15]	Pacchioni G. Valleytronics with a twist. Nat Rev Mater, 2020, 5, 480 doi: 10.1038/s41578-020-0220-2
[16]	Shao G L, Xue X X, Liu X, et al. Twist angle-dependent optical responses in controllably grown WS₂ vertical homojunctions. Chem Mater, 2020, 32, 9721 doi: 10.1021/acs.chemmater.0c03413
[17]	Cao T, Wang G, Han W P, et al. Valley-selective circular dichroism of monolayer molybdenum disulphide. Nat Commun, 2012, 3, 887 doi: 10.1038/ncomms1882
[18]	Zhang T Y, Zhao S W, Wang A R, et al. Electrically and magnetically tunable valley polarization in monolayer MoSe₂ proximitized by a 2D ferromagnetic semiconductor. Adv Funct Mater, 2022, 32, 2204779 doi: 10.1002/adfm.202204779
[19]	Li X T, Liu Z D, Liu Y H, et al. All-electrical control and temperature dependence of the spin and valley Hall effect in monolayer WSe₂ transistors. ACS Appl Electron Mater, 2022, 4, 3930 doi: 10.1021/acsaelm.2c00599
[20]	Plechinger G, Nagler P, Kraus J, et al. Identification of excitons, trions and biexcitons in single-layer WS₂. Phys Status Solidi RRL, 2015, 9, 457 doi: 10.1002/pssr.201510224
[21]	Gong Y Y, Carozo V, Li H Y, et al. High flex cycle testing of CVD monolayer WS₂ TFTs on thin flexible polyimide. 2D Mater, 2016, 3, 021008 doi: 10.1088/2053-1583/3/2/021008
[22]	Wu K, Zhong H X, Guo Q B, et al. Revealing the competition between defect-trapped exciton and band-edge exciton photoluminescence in monolayer hexagonal WS₂. Adv Opt Mater, 2022, 10, 2101971 doi: 10.1002/adom.202101971
[23]	Hu Z L, Avila J, Wang X Y, et al. The role of oxygen atoms on excitons at the edges of monolayer WS₂. Nano Lett, 2019, 19, 4641 doi: 10.1021/acs.nanolett.9b01670
[24]	Kim M S, Yun S J, Lee Y J, et al. Biexciton emission from edges and grain boundaries of triangular WS₂ monolayers. ACS Nano, 2016, 10, 2399 doi: 10.1021/acsnano.5b07214
[25]	Chow P K, Jacobs-Gedrim R B, Gao J, et al. Defect-induced photoluminescence in monolayer semiconducting transition metal dichalcogenides. ACS Nano, 2015, 9, 1520 doi: 10.1021/nn5073495

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K. Mahmood, N. Amin, A. Ali, M. Ajaz un Nabi, M. Imran Arshad, M. Zafar, M. Asghar. Enhancement of phosphors-solubility in ZnO by thermal annealing[J]. Journal of Semiconductors, 2015, 36(12): 123001. doi: 10.1088/1674-4926/36/12/123001

K. Mahmood, N. Amin, A. Ali, M. Ajaz un Nabi, M. Imran Arshad, M. Zafar, M. Asghar. Enhancement of phosphors-solubility in ZnO by thermal annealing[J]. J. Semicond., 2015, 36(12): 123001. doi: 10.1088/1674-4926/36/12/123001.

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Received: 02 November 2022 Revised: 30 November 2022 Online: Accepted Manuscript: 09 December 2022Uncorrected proof: 19 December 2022Published: 14 January 2023

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K. Mahmood, N. Amin, A. Ali, M. Ajaz un Nabi, M. Imran Arshad, M. Zafar, M. Asghar. Enhancement of phosphors-solubility in ZnO by thermal annealing[J]. Journal of Semiconductors, 2015, 36(12): 123001. doi: 10.1088/1674-4926/36/12/123001 ****K. Mahmood, N. Amin, A. Ali, M. Ajaz un Nabi, M. Imran Arshad, M. Zafar, M. Asghar. Enhancement of phosphors-solubility in ZnO by thermal annealing[J]. J. Semicond., 2015, 36(12): 123001. doi: 10.1088/1674-4926/36/12/123001.

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Siwen Zhao, Gonglei Shao, Zheng Vitto Han, Song Liu, Tongyao Zhang. Gate tunable spatial accumulation of valley-spin in chemical vapor deposition grown 40°-twisted bilayer WS₂[J]. Journal of Semiconductors, 2023, 44(1): 012001. doi: 10.1088/1674-4926/44/1/012001 ****

S W Zhao, G L Shao, Z V Han, S Liu, T Y Zhang. Gate tunable spatial accumulation of valley-spin in chemical vapor deposition grown 40°-twisted bilayer WS₂[J]. J. Semicond, 2023, 44(1): 012001. doi: 10.1088/1674-4926/44/1/012001

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Gate tunable spatial accumulation of valley-spin in chemical vapor deposition grown 40°-twisted bilayer WS₂

DOI: 10.1088/1674-4926/44/1/012001

1.
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110010, China
2.
Institute of Chemical Biology and Nanomedicine, State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China
3.
State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Opto-Electronics, Shanxi University, Taiyuan 030006, China
4.
Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China

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Siwen Zhao：received his BS degree from Shandong University and then finished his Ph.D. degree from the Department of Physics in the University of Science and Technology of China. He held a position of postdoctoral fellow in Shenzhen University from 2019 to 2022. Currently, he is working as a Research Associate in the Institute of metal research, Chinese Academy of Sciences. His recent research interests focus on the electronic transport properties and potential applications of nanometer-scale devices based on novel 2D materials and their heterostructures
Song Liu：is a full Professor at the College of Chemistry and Chemical Engineering, Hunan University. He received his BS degree from Nankai University and then finished his Ph.D. degree from Peking University. After that he worked at Case Western Reserve University and National University of Singapore as postdoctoral fellow. His research interests focus on the low dimensional materials and functional devices, as well as the applications in energy and biosensors
Tongyao Zhang：is a lecturer at the Institute of opto-electronics, Shanxi University. He received his BS degree and Ph.D. degree from Shanxi University. He mainly focuses on low-temperature microscopic spectroscopy of atomically thin van der Waals heterostructures by optical probe and optical manipulation
Corresponding author: liusong@hnu.edu.cn; tongyao_zhang@sxu.edu.cn
Received Date: 2022-11-02
Revised Date: 2022-11-30

Available Online: 2022-12-09

Abstract

Abstract

The emerging two-dimensional materials, particularly transition metal dichalcogenides (TMDs), are known to exhibit valley degree of freedom with long valley lifetime, which hold great promises in the implementation of valleytronic devices. Especially, light–valley interactions have attracted attentions in these systems, as the electrical generation of valley magnetization can be readily achieved — a rather different route toward magnetoelectric (ME) effect as compared to that from conventional electron spins. However, so far, the moiré patterns constructed with twisted bilayer TMDs remain largely unexplored in regard of their valley spin polarizations, even though the symmetry might be distinct from the AB stacked bilayer TMDs. Here, we study the valley Hall effect (VHE) in 40°-twisted chemical vapor deposition (CVD) grown WS₂ moiré transistors, using optical Kerr rotation measurements at 20 K. We observe a clear gate tunable spatial distribution of the valley carrier imbalance induced by the VHE when a current is exerted in the system.
- transition metal dichalcogenides,
- valleytronic devices,
- light–valley interactions,
- valley Hall effect

Supplementary materials to this article can be found online at https://doi.org/10.1088/1674-4926/44/1/012001.

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1. Introduction

Zinc oxide is a very important material for electronic and optoelectronic devices because of its excellent properties of direct and wide band gap and large exciton binding energy^{[1, 2]}. The greatest challenge for these applications,however,remains the fabrication of reliable and stable p-type ZnO with low resistivity and high carrier concentration^[3]. This p-type doping issue is related to the formation and compensation effects of native donor defects such as O vacancies and Zn interstitials,low solubility of p-dopants and higher activation energy of acceptors in the band gap of ZnO^{[4, 5, 6]}. It is reported in the literature that even nitrogen and phosphorous are the most favorable p-type dopants in ZnO,but stable p-type conductivity is hard to achieve due to the low solubility^[7]. The solubility limit for acceptor impurities in ZnO,as well as other wide-gap materials suffering from doping asymmetry,can be enhanced above the thermodynamic limit by employing ``non-equilibrium'' conditions. One way to improve the dopant solubility is through adjusting dopant chemical potential $\mu$ A. The chemical potential of an acceptor dopant in the ZnO matrix determines the dopant solubility^[8]. Therefore,raising this potential would increase the solubility of the dopant coupled with avoidance of precipitate formation. High temperature annealing is a very efficient way to facilitate the enhancement of P concentration. Furthermore,high temperature annealing can activate the P atoms in ZnO,suppress the formation of P $_{\rm O}$ defects and reduce the density of oxygen vacancies^[9]. Therefore,a high temperature annealing study would be very helpful to grow stable p-type ZnO.

In this paper,we have achieved high solubility of phosphorous in bulk ZnO by using high temperature annealing conditions. The ZnO pellets were annealed from 500 to 1000 ℃ in a programmable furnace; it was found that the maximum concentration of P occurred in a sample annealed at 800 ℃. The results were explained with the help of XRD,PL and Hall measurements.

2. Experimental

ZnO pellets were synthesized by the conventional sintering technique in an atmospheric furnace using a zinc oxide powder (99.99%). The ZnO powders were first milled in an agate mortar and heated in air at an annealing temperature of 300 ℃ for 2 h to evaporate the water and remove the organic residuals. The obtained powders were then pressed into pellet discs (of about 1 mm in thickness and 8 mm in diameter). A drop of phosphors dopant P430 was sprayed on the surface of pellets and then subjected to the annealing process. The pellets annealed at different temperatures from 500 to 1000 ℃,keeping a step of 100 ℃ using a programmable furnace for one hour.

The characterization of annealed samples was done by using following equipments; SEM/EDX (3000 Hitachi),PL/Raman (Photon Systems) having laser wavelength 248 nm,Hall effect by Ecopia 3000 and XRD (miniflex Rigaku). All the measurements were performed at room temperature.

3. Results and discussion

3.1 EDX measurements

shows the EDX results of P-doped ZnO annealed at different temperatures from 500 to 1000 ℃. The figure shows that the concentration of phosphorous increases with annealing temperature up to an annealing temperature of 800 ℃ and again decreases for samples annealed at higher temperatures. Moreover,the concentration of oxygen content in the samples also increases with annealing temperature. The O-rich growth conditions suppress the formation of P $_{\rm O}$ acceptor defects and instead increase the probability of generating Zn vacancies and O-interstitials due to the Zn deficiency conditions. Among them,Zn vacancy-related defects result in the rapid incorporation of P atoms in the Zn sites,and the ultimate result is the formation of a P $_{\rm Zn}$ -2V $_{\rm Zn}$ acceptor complex^[10]. The formation mechanism of this acceptor complex has already been reported by many authors^{[11, 12]}. To verify the p-type conductivity of annealed samples,we have performed Seebeck measurements. The results suggested that samples annealed at 800 and 900 ℃ showed p-type conductivity and the rest all showed n-type conductivity. This result confirmed that the maximum diffusion of P atoms into ZnO pellets occurred at an annealing temperature of 800 ℃.

Figure 1. EDX measurements of bulk ZnO annealed at different temperatures from 500 to 1000 ℃.

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3.2 XRD measurements

Typical XRD patterns of the P-doped ZnO pellets sintered at different temperatures (500 to 1000 ℃ with a step of 100 ℃) for one hour demonstrated `2 $\theta$ values' of 8 diffraction peaks corresponding to the ZnO (100),(002),(101),(102),(110),(103),(200) and (112) planes,respectively. A comparison with the JCPD 36-1451 Card confirmed the formation of hexagonal zinc oxide^{[13, 14]}. For clarity,we have displayed only the dominant diffraction peaks corresponding to the (002) plane,in . We clearly see that the 2 $\theta$ value of the major peak of the representative ZnO samples varies with the sintering temperature. The 2 $\theta$ value of the peak increases and reaches 34.6 $^\circ$ at the annealing temperature of 800 ℃ shown in the inset of . According to the size consideration of P,an increase in lattice constant is expected when a P atom occupies an O site (smaller in size than P); otherwise,the lattice constant should decrease when a P atom occupies a Zn site (larger in size than P)^[15]. Practically,the (002) plane of the ZnO shifts towards a higher 2 $\theta$ value with the sintering temperatures up to 800 ℃ and thus yields a smaller lattice constant. Henceforth,we will refer the filling of Zn site with P as an A-type shift. Accordingly,the information from the literature suggest that a P atom while occupying a Zn site may generate two Zn vacancies and hence produces a shallow acceptor P $_{\rm Zn}$ -2V $_{\rm Zn}$ complex above the valence band maximum.

Figure 2. (Color online) X-ray diffraction pattern of P-doped bulk ZnO annealed from 500 to 1000 ℃. Inset demonstrated the effect of annealing temperature on the peak position of ZnO (002) plane.

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3.3 PL spectroscopy

The room temperature PL measurements were carried out to examine the effect of annealing on the optical properties of the P-doped ZnO pellets. Figure3 compares the photoluminescence spectra of doped samples measured at room temperature. All samples consist of a band to band emission at 3.29 eV,but samples annealed at 800,900 and 1000 ℃ have an additional peak at 3.2 eV^[16]. This additional peak is related with the donor acceptor pair,where the acceptor is the P atom.

Figure 3. (Color online) Photoluminescence spectra of P-doped ZnO powder annealed at 500-1100 ℃.

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3.4 Hall measurements

The Hall measurements were performed to calculate the carrier concentration of P-doped samples and shown in . The graph shows that the hole concentration is maximum for a sample annealed at 800 ℃. The observed result can be explained as follows: as the annealing temperature increases the P doping increases,which formed an acceptor complex level with V $_{\rm Zn}$ ,therefore the carrier concentration increases. All other samples show the n-type conductivity (not shown here).

Figure 4. Annealing temperature versus carrier concentration graph.

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4. Conclusion

In conclusion,we have successfully enhanced the phosphorous diffusion in ZnO pellets to achieve p-type conductivity. ZnO pellets were doped with P using high temperature annealing conditions. The maximum diffusion of P atoms occurred at the annealing temperature of 800 ℃,confirmed by EDX measurements. The shifting of the (002) ZnO plane towards higher 2 $\theta$ angles in XRD and emerging of donor acceptor pair in PL spectra at the annealing temperature of 800~℃ strongly suggested that maximum diffusion of P atoms occurred at the annealing temperature of 800 ℃.

Acknowledgements

Authors are thankful to the Higher Education Commission (HEC) of Pakistan for the financial assistance under project # IPFP/HRD/HEC/2014/2016.

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