Article pubs.acs.org/JPCC
Reversible n‑Type Doping of Graphene by H2O‑Based Atomic-Layer Deposition and Its Doping Mechanism Li Zheng,†,‡ Xinhong Cheng,*,† Zhongjian Wang,† Chao Xia,†,‡ Duo Cao,†,‡ Lingyan Shen,†,‡ Qian Wang,†,‡ Yuehui Yu,† and Dashen Shen§ †
State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § University of Alabama in Huntsville, Huntsville, Alabama 35899, United States ABSTRACT: The pre-H2O treatment and Al2O3 film growth under a two-temperatureregime mode in an oxygen-deficient atomic layer deposition (ALD) chamber can induce ntype doping of graphene, with the enhancement of electron mobility and no defect introduction to graphene. The main mechanism of n-type doping is surface charge transfer at graphene/redox interfaces during the ALD procedure. More interestingly, this n-type doping of graphene is reversible and can be recovered by thermal annealing, similar to hydrogenated graphene (graphane). This technique utilizing pre-H2O treatment and an encapsulated layer of Al2O3 achieved in an oxygen-deficient ALD chamber provides a simple and novel route to fabricate n-type doping of graphene.
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INTRODUCTION Graphene has triggered tremendous scientific and technological interests due to its outstanding properties. The exclusive properties of graphene, such as ultrahigh charge carrier mobility,1 gigantic thermal conductivity,2 and exceptional mechanical strength,3 along with the two-dimensional structure and large-scale production, make it attractive for high-speed electronic devices.4−6 Despite the aforementioned success, graphene in its pristine form is insufficient to satisfy diverse specific demands.5,7,8 For instance, field-effect transistors (FETs) using graphene as a semiconducting channel between the source and drain suffer from the absence of a bandgap.7,8 Chemical modification or doping is one subject that draws attention. It can tune carrier types or Fermi energy levels of graphene and even open a band gap.9,10 Unfortunately, the twodimensional sp2 hybrid carbon structure of the graphene surface makes it challenging to induce any chemical modification or doping without alteration of its idealized properties.5,11 In addition, the task to obtain a stable n-type graphene transistor at ambient condition is generally more difficult to achieve than its p-type counterpart. To date, three typical chemical modification and doping strategies (covalent functionalization, substitutional heteroatom doping, and charge-transfer doping) have been introduced for n-type doping of graphene.5,12−14 Covalent functionalization can be achieved via photochemical, electrochemical, and plasma treatments. It alters the local bonding environment of graphene from sp2 to sp3 hybrid structures and results in the deterioration of carrier mobility and electrical/thermal conductivity.12 Substitutional heteroa© 2015 American Chemical Society
tom doping has proven itself as an efficient route to modify the atomic-scale structures, surface energy, and chemical reactivity of graphene.13−15 Nevertheless, it is usually accompanied by defect formation in graphene structures, which may deteriorate the ballistic charge mobility and superstrong mechanical properties. Interface charge-transfer doping is a new class of doping relying on spontaneous charge transfer from physisorbed species.5 Various kinds of dopants, including gas molecules, polymers, metals, and metal oxides, have been exploited for effective charge transfer to graphene. Schedin et al. have examined that NH3 could cause strong charge-transfer doping of graphene.16 However, volatile gas is unsuitable for stable doping in practical applications. In this regard, the charge transfer doping from organic molecules or polymers shows potential for simple and stable doping. Dong et al. have verified that the doping level of graphene films can be modulated by aromatic compounds, and they achieved air-stable n-type doping of graphene.17 Unfortunately, organic dopants and polymers lack chemical stability. Therefore, they are not suitable for standard lithography, which is the conventional device-fabrication process involved. To address this technological issue, inorganic materials, such as metals and metal oxides, have been investigated for charge-transfer doping of graphene. For example, McCreary et al. reported substantial ntype doping of graphene by depositing a submonolayer of Received: November 18, 2014 Revised: March 2, 2015 Published: March 5, 2015 5995
DOI: 10.1021/jp511562t J. Phys. Chem. C 2015, 119, 5995−6000
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Figure 1. Fabricating procedure of n-type doping of graphene by H2O-based ALD.
Raman spectroscopy was first carried out to investigate the H2O-based ALD-induced n-type doping of graphene and its reversibility. The main features in the Raman spectrum of graphene are the G and 2D peaks, lying at around 1593 and 2690 cm−1, respectively. The G peak corresponds to optical E2g phonons at the Brillouin zone center (Γ), whereas the 2D peak originates from a process where momentum conservation is satisfied by two phonons with opposite wave vectors.22 Therefore, no defects are required for the activation of the 2D peak, and it thus always presents. In addition, the 2D peak shows a different response to electrons and holes, making it a sensitive parameter to monitor the doping of graphene.19 As shown in Figure 2, after pre-H2O treatment and ALD-Al2O3
metallic Ti using a molecular beam epitaxy system.18 In addition, Ho et al. demonstrated an n-type graphene transistor by spin coating a kind of sol−gel TiOx solution.19 In our previous work, we tried to deposit Al2O3 films on graphene by ALD without the assistance of a transition layer or surface functionalization.20 By controlling gas−solid physical adsorption between H2O molecules and graphene through the optimization of pre-H2O treatment and two-step temperature growth, we directly grew uniform and compact Al2O3 films onto graphene by ALD. During the experimental process, we found that the doping type of graphene could be transformed, and interface charge-transfer doping might be the possible doping mechanism. More interestingly, this n-type doping of graphene could be reversed through thermal annealing. Consequently, further work was performed to address this issue. In this work, an innovative approach to fabricate n-type graphene with air stability and reversibility was achieved by atomic layer deposition (ALD). The pre-H2O treatment in an oxygen-deficient ALD chamber resulted in n-type doping of graphene with enhanced electron mobility, and the subsequent atomic-layer-deposited Al2O3 could act as an encapsulated layer to ensure its air stability. This H2O-based ALD-induced n-type doping of graphene could be reversed after thermal annealing, which was similar to graphane.21 The reversible n-type doping of graphene was confirmed by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrical measurements.
Figure 2. Raman spectroscopy measurement of graphene at different conditions: pristine graphene, graphene with pre-H2O treatment and ALD-Al2O3 deposition, and graphene with pre-H2O treatment and ALD-Al2O3 deposition after RTA.
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RESULTS AND DISCUSSION Figure 1 shows the technological procedure of H2O-based ALD-induced n-type doping of graphene. After transferring graphene on SiO2/Si, graphene samples were put into an ALD chamber, and a few cycles of H2O were introduced onto graphene. The physically absorbed H2O molecules could also act as deposited sites for subsequent ALD-Al2O3 growth. In order to obtain uniform and pinhole-free Al2O3 film on graphene, two temperature regimes were utilized for Al2O3 growth.20,21 A 4 nm thickness of Al2O3 was first deposited onto graphene at 100 °C, and the chamber was subsequently elevated to 200 °C to deposit another 5 nm thickness of Al2O3. The Al2O3 film could act as an encapsulated layer to maintain the air stability of n-type doped graphene. Moreover, due to the excellent insulating property of Al2O3, it could also play a key role in electrical applications of graphene such as GFETs. After Raman, XPS, and electrical measurements, graphene samples were treated with rapid thermal annealing (RTA) at 400 and 800 °C for 30 s, respectively. We use graphene/H2O and graphene/H2O/RTA to represent the graphene samples with pre-H2O treatment and ALD-Al2O3 deposition before and after RTA, respectively.
growth, the 2D peak of graphene shifted down from 2690 to 2685 cm−1, due to the effect of the Fermi level shift on the photon frequencies as a result of n-type doping of graphene.19,22 In addition, the full width at half-maximum (fwhm) values of the 2D peak shifted up from 43 to 47 cm−1. Both of these Raman spectroscopy results including the 2D peak left-shifting and blunting were consistent with those observed in typical n-type doping of graphene.19,20,23 Notice that the 2D peak shifted up from 2685 to 2689 cm−1, while the fwhm values of the 2D peak shifted down from 47 to 45 cm−1 after the n-type doped graphene was treated with RTA at 800 °C, indicating the reversible trend of H2O-based ALD-induced n-type doped graphene. The possible reason for n-type doping reversion is the evaporation of H2O molecules adsorbed to graphene during the ALD process, which we will discuss in detail later. It was worth mentioning that Al2O3 applied external strain on graphene, and RTA could cause strong adhesion of Al2O3 on graphene, which gave rise to the nonadiabatic removal of the Kohn anomaly from the Γ point and resulted in the upshift of the G peak. For this reason, the intensity radio of the 5996
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The Journal of Physical Chemistry C 2D to G peak could not be a criterion to identify the doping type of graphene after RTA. The defect-related bands of graphene in the Raman spectra were D and D′ peaks located at around 1350 and 1620 cm−1, respectively. The D peak is caused by breathing-like modes and requires a defect for its activation via an intervalley double resonance, while the D′ peak occurs via an intravalley double-resonance process in the presence of defects.22 No obvious change of D and D′ peaks was observed after both ALD and RTA processes, indicating H2O-based ALD-induced n-type doping and RTA-induced reversibility mainly occurred through charge transfer doping and did not introduce defects into graphene. In addition to the Raman measurements, the H2O-based ALD-induced reversible n-type doped graphene was also detected by the XPS spectra. Since the information depth of XPS is only 2−3 nm, part of the Al2O3 film on graphene was etched for accurate measurements. All the XPS peaks were calibrated with the Al 2p peak. As shown in Figure 3, the
Figure 4. (a) Schematic of a GFET with pre-H2O treatment and ALDAl2O3 deposition. (b) A top view of a GFET structure. (c) The gatedependent conductivity of GFETs at different conditions: pristine graphene, graphene with pre-H 2 O treatment, and ALD-Al2O 3 deposition and graphene with pre-H2O treatment and ALD-Al2O3 deposition after RTA (at 400 and 800 °C, respectively).
Dirac gate voltage was observed, and the higher RTA temperature led to a more positive shift of the Dirac gate voltage. The Dirac point of the graphene samples shifted from 60 V (pristine) to −18 V (with pre-H2O treatment), to 40 V (after RTA at 400 °C) and to 55 V (after RTA at 800 °C), and the corresponding carrier concentrations as estimated by n = Cg·Vg/e were 4.3 × 1012 cm−2 (pristine/hole doping), 1.3 × 1012 cm−2 (with pre-H2O treatment/electron doping), 2.9 × 1012 cm−2 (after RTA at 400 °C/hole doping), and 4 × 1012 cm−2 (after RTA at 800 °C/hole doping), respectively. Electron and hole mobility of graphene could be extracted by measuring the slopes of the conductivity curves away from the Dirac points. No decline in hole mobility (extracted from the left branch of conductivity curves) was observed after pre-H2O treatment, while the electron mobility (extracted from the right branch of conductivity curves) was significantly enhanced. This phenomenon differed considerably from those obtained in the covalent functionalization doping or substitutional heteroatom doping of graphene, in which electron mobility decreased seriously after doping.12,13 Impurity density reduction induced by self-cleaning of ALD was an important factor to explain the enhanced electron mobility of graphene. During the ALD process, the residual impurity attached to graphene such as OH− was eliminated by self-cleaning of ALD, leading to scattering sites reduction and electron mobility enhancement. After RTA, the electron mobility decreased. RTA could cause strong adhesion of Al2O3 on graphene, which resulted in the enhancement of remote oxide phonon scattering sites and the reduction of electron mobility.26 The obtained experimental result that H2O-based ALD can induce n-type doping of graphene is different from some other reports. For example, Yavari et al. reported that graphene was p-type doped with the existence of H2 O at ambient conditions.27 This kind of p-type doping was caused by H2O/O2 redox generated on the graphene surface at ambient conditions rather than H2O itself. Here we apply Marcus−
Figure 3. XPS spectra of C 1s peaks of graphene at different conditions: pristine graphene, graphene with pre-H2O treatment and ALD-Al2O3 deposition, and graphene with pre-H2O treatment and ALD-Al2O3 deposition after RTA.
binding energy of the C 1s peak of the pristine graphene corresponding to pure sp2-hybridized states was centered at 284.6 eV. After pre-H2O treatment, the C 1s peak shifted up from 284.6 to 285.1 eV, and its bandwidth was broadened. The observed energy shift (0.5 eV) indicated the n-type doping effect of H2O-based ALD on graphene.19,23 When the graphene/H2O samples were treated with RTA at 800 °C, the C 1s peak shifted down from 285.1 to 284.6 eV, representing the reversible H2O-based ALD-induced n-type doping of graphene by ALD. For the purpose of further confirming the reversible n-type doping of graphene induced by pre-H2O treatment in the ALD process, back-gated GFETs were fabricated to investigate the transport property of graphene, where Dirac points (the gate dependence of minimum drain current) shift depending on electron or hole doping levels.24,25 A schematic device diagram and a top view of a GFET structure are shown in Figures 4a and 4b, respectively. Figure 4c shows the gate-dependent conductivity of GFETs with pre-H2O treatment and ALDAl2O3 deposition. For comparison, the pristine graphene transistor deposited on SiO2/Si was also fabricated and exhibited a typical p-type transporting behavior. After preH2O treatment and ALD-Al2O3 deposition, the Dirac points shifted toward negative gate voltages, representing a typical ntype transport behavior. When RTA was performed to graphene samples, a clear trend toward a positive shift of the 5997
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Figure 5. (a) Doping mechanism of graphene: left, schematic of the H2O/O2 redox couple density of states (DOS) for an equivalent concentration of oxidizing and reducing species; right, comparison with the DOS of graphene after the H2O-based ALD process. Electron is transferred to graphene from the redox. (b) The gate-dependent conductivity of GFETs at different conditions: pristine graphene, graphene with pre-H2O treatment, and ALD-Al2O3 deposition at 100 °C/200 °C, and with pre-H2O treatment and ALD-Al2O3 deposition at 100 °C. (c) XPS spectra of O 1s of Al2O3 films deposited on graphene at 100 and 100 °C/200 °C (inset).
if cox is significantly reduced, according to the Nernst equation, the redox Fermi level Eredox shifts up and drives the electrons from the occupied levels in the redox toward the unoccupied states of graphene.30 This process sets the conditions for the following redox reaction
Gerischer theory to explain the electron transferring difference between H 2 O/O 2 redox and graphene when the O 2 concentration (cox) of the redox is significantly changed.28−30 The Fermi level of the redox relative to the vacuum level is given by the Nernst equation30 0 Eredox = ESHE + Eredox −
akBT [log(cox ) − 4pH] zF
H 2O ⇌ O2 (aq) + 4H+ + 4e− (graphene) (1)
(2)
consuming reactive oxygen species such as OH− and generating H+, which fixes a net positive charge on the SiO2/Si substrate surface. There are two methods to facilitate reaction 2 to the forward direction. One is to put the graphene sample into an oxygen-deficient environment with a vacuum system to pump O2 (e.g., ALD chamber); the other one is to increase the amount of H2O in the redox to reduce cox (e.g., pre-H2O treatment). Therefore, H2O-based ALD is an effective way to dope graphene into the n-type. In addition, the subsequent deposited Al2O3 acts as an encapsulated layer and isolates graphene from the atmospheric environment, guaranteeing that the H2O-based ALD-induced n-type doping of graphene can be detected. After RTA, the H2O absorbed onto graphene during the H2O-based ALD process is reduced due to evaporation, leading to cox rising and suppression of reaction 2. As a result,
where (akBT/zF) = 0.0148 eV at room temperature; ESHE (∼4.44 eV) is the standard hydrogen electrode (SHE) potential relative to the vacuum level; E0redox (∼−10.13 eV) is the standard electrode potential of the reaction versus SHE; z is the number of electrons transferred in the redox; and kB and F are the Boltzmann’s constant and Faraday constant, respectively. By Henry’s law with an O2 partial pressure in air of 0.21 bar,30 the Fermi level of the redox Eredox obtained from eq 1 is −5.66 eV at pH = 0 and −4.83 eV at pH = 14. As shown in Figure 5a, the Fermi level of graphene (−4.6 eV) lies above the electrochemical potential of the H2O/O2 redox couple (−5.3 eV at pH = 6 for a realistic acidity of water in air), providing a strong driving force for electron transfer from the Fermi level of graphene to the unoccupied state of the redox. On the contrary, 5998
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The Journal of Physical Chemistry C the Fermi level of the redox shifts down, which forces a shift of the graphene Fermi level well into its valence band and recovers graphene into p-type. In order to verify that the n-type doping of graphene was induced by H2O-based ALD rather than oxygen vacancies in Al2O3 and its reversibility was due to evaporation of H2O rather than reduction of oxygen vacancies during the RTA process, Al2O3 deposited on graphene at low temperature (100 °C) was also performed. If H2O during the ALD process induces graphene to n-type doping, this effect should be weakened by decreasing growth temperature since reaction 2 is an endothermic reaction according to Lange’s Chemistry Handbook Version 15.31 As illustrated in Figure 5b, the Dirac point of the graphene shifted from 60 to 10 V when the ALD chamber temperature was 100 °C, indicating the p-type doping of pristine graphene was suppressed by pre-H2O treatment. However, 100 °C was not enough to turn graphene into n-type doping due to the endothermic reaction of reaction 1. In addition, for the sample grown at 100 °C, the O 1s peak centered at 531.7 eV had a slight asymmetry, and further deconvolution revealed two distinct components (shown in Figure 5c), the stronger peak locating at 531.4 eV originating from Al−O bonds and the other weak peak 532.9 eV associated with Al−O−H hydroxyl groups due to the incomplete reaction of trimethylaluminum (TMA) and H2O at low temperature (100 °C). The O/Al atomic ratio in this growth mode was 1.34. As for Al2O3 deposited on graphene at two temperature regimes (100 °C/200 °C), the Al−O bonds related peak upshifted, while the Al−O−H bonds related peak downshifted (shown in the inset of Figure 5b). The O/Al atomic ratio turned to be 1.45, indicating oxygen vacancy reduction and property enhancement of Al2O3 on graphene. Al2O3 deposited at 100 °C/200 °C had less oxygen vacancies, while the n-type doping effect was more prominent, compared with that grown at 100 °C. Therefore, the oxygen vacancies existing in Al2O3 films were not the major factor for n-type doping of graphene, and pre-H2O treatment in the ALD process induced the n-type doping of graphene and its reversibility.
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CONCLUSION
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AUTHOR INFORMATION
Article
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ACKNOWLEDGMENTS
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REFERENCES
This work was supported by the National Natural Science Foundation of China (Grant No. 11175229). We would like to thank Prof. Zengfeng Di, Dr. Haoran Zhang, and Dr. Xiaohu Zheng and for their generous help.
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In conclusion, the pre-H2O treatment in the oxygen-deficient ALD chamber can induce n-type doping of graphene with enhanced electron mobility, and the subsequent deposited Al2O3 can act as an encapsulated layer to ensure its air stability. The Fermi level of redox absorbed on the graphene surface upshifts through pre-H2O treatment during the ALD process and drives the electrons from the occupied level in the redox toward the unoccupied level of graphene, leading to n-type doping of graphene. In addition, the n-type doping of graphene is reversible and can be recovered by thermal annealing. This technique is an innovative approach to fabricate n-type-doped graphene and has considerable applications in future development of graphene-based nanoelectronics.
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[email protected]. Tel.: +862162511070. Notes
The authors declare no competing financial interest. 5999
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