An ultraviolet photoelectron spectroscopy study on bandgap broadening of epitaxial graphene on SiC with surface doping

Journal Pre-proof An ultraviolet photoelectron spectroscopy study on bandgap broadening of epitaxial graphene on SiC with surface doping Yanfei Hu, Jialin Liu, Wentao Dou, Kaili Mao, Lixin Guo, Laiyuan Chong, Jichao Hu, Hao Yuan, Yanjing He, Hui Guo, Yuming Zhang PII:

S0008-6223(19)31058-9

DOI:

https://doi.org/10.1016/j.carbon.2019.10.043

Reference:

CARBON 14705

To appear in:

Carbon

Received Date: 21 July 2019 Revised Date:

17 October 2019

Accepted Date: 18 October 2019

Please cite this article as: Y. Hu, J. Liu, W. Dou, K. Mao, L. Guo, L. Chong, J. Hu, H. Yuan, Y. He, H. Guo, Y. Zhang, An ultraviolet photoelectron spectroscopy study on bandgap broadening of epitaxial graphene on SiC with surface doping, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.10.043. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

An Ultraviolet Photoelectron Spectroscopy study on bandgap broadening of epitaxial graphene on SiC with surface doping Yanfei Hua, Jialin Liua, Wentao Doub, Kaili Maoc, Lixin Guod, Laiyuan Chonga, Jichao Hue, Hao Yuana,∗, Yanjing Hea, Hui Guoa,* , Yuming Zhanga

a School of Microelectronics, Key Laboratory of Wide Band-Gap Semiconductor Materials and Devices, Xidian University, Xi’an 710071, CHINA

b SICC Co., Ltd, Jinan 250018, CHINA

c Department of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, CHINA

d School of Physics and Optoelectronic Engineering, Xidian University, Xi’an 710071, CHINA

e Department of Electronic Engineering, Xi’an University of Technology, Xi’an 710048, CHINA

ABSTRACT

The bandgap of epitaxial graphene thermally grown on 4H-SiC(0001) can be widened via a novel charge transfer mechanism by modifying the buffer layer and surface of SiC with substrate doping before epitaxial graphene growth. High-resolution photoemission spectroscopy base on synchrotron radiation reveal that the bandgap broadening of epitaxial graphene is attributed to charge transfer ∗ *

Corresponding author. Tel: +86 029 88201421. E-mail: [email protected]. Corresponding author: Tel: +86 029 88201644. E-mail: [email protected] 1

from the substrate to epitaxial graphene, and both donor-acceptor and bulk atoms injected in the SiC make the bandgap of EG on those substrates broaden. This novel surface charge transfer doping mechanism via substrates doping will provide a simple and effective approach to nondestructively broad the bandgap of epitaxial graphene for future electronics application.

1. Introduction

Epitaxial graphene thermally grown on SiC (EG) is of great potentials in the advance of graphene device as it is ready for processing electronic devices and no transfer is required; EG is compatibility with existing Si technology and can be directly used to fabrication ICs on the whole wafer via traditional process; EG is favorable for its well-established SiC substrate to high-frequency electronics, radiation hard devices, and light-emitting devices. What’s more, Charges transferring from the substrate due to the special buffer layer makes EG be heavily doped and exhibit special metallic features and high carrier mobilities[1], as a result, EG has a bandgap about 0.26 eV[2, 3] which is necessary to the on-off properties of GFETs[1] Additionally, for GNRs grown on SiC, the carrier mobility has no significant negative correlation to bandgap width. J. Baringhaus et al demonstrate that GNRs (40 nm wide) grown on SiC has a huge µ (~6 × 10

)[4].

Wafer-size EG on SiC is supposed to the most promising approach for graphene devices and ICs.

But, so far, the bandgap of EG is insufficient to satisfy the fabrication of large scale integrated circuit. Bandgap Broaden still is the near-term target for EG application.

2

Several technologies have been developed to broaden the bandgap of EG, such as patterning into GNRs, in situ doping, atomic adsorption and so on. Palacio et al first broaden the bandgap of EG by patterning the epitaxial layer into GNRs[5]. Recently, Ichinokura et al fabricate EG samples with a 0.5 eV bandgap by intercalating Ga atoms into the buffer layer between epitaxial graphene and SiC[6]. Varchon, F et al demonstrate that electrons could transfer from SiC substrate to EG via the special intermediate layer structure, as a result, the electronic structure of EG changed[7]. Technology difficulty and transfer costs are the two main barriers confronted by the above-mentioned bandgap engineering approaches. In this work, the bandgap broadening of EG via a novel charge transfer mechanism by modifying the buffer layer and surface of SiC with the substrate doping before EG growth is studied. Injecting different impurities into near-surface of SiC before EG growth would alter the structure of SiC surface and consequently change the situation of charge transfer from SiC substrate to EG, resulting in the bandgap variation of EG. On the other side, selective doped region of SiC has different EG growth threshold from undoped area, and, GNRs could be fabricated via selective doping the near-surface of SiC. This indirect bandgap broadening approach will become the main method to realize the gap engineering of graphene.

2. Experimental

2.1 Research design

Regular surface imperfection and structure variation are needed to improve EG nucleation sites and properties[3, 8-10]. Appropriate ion implantation is an important method to perform surface modification and alter the symmetry of the surface structure. Injected C atoms in SiC could serve as 3

a carbon source for EG growth, which could reduce excessive consumption of Si-C layer, thereby guarantee interface smooth for EG growth. What’s more, rational distribution of surface imperfections would form after the C implantation, which alters the lattice stability at SiC surface, thereby reducing the activation energy of Si at the surface and the threshold temperature of EG growth. In this study, EG were first fabricated on substrates with surface doping listed in table 1. Monte Carlo (SRIM) calculations of the implanted C profiles and the damage of the implanted SiC are elaborated in section III of supplementary information.

Table 1 Sample sets and their substrate doping condition

Samples

Implantation dose

Implantation energy

1#

5×1013 cm-2

30 keV

2#

5×1013 cm-2

35 keV

3#

5×1013 cm-2

40 keV

4#

5×1014 cm-2

30 keV

The EG on SiC without doping is also fabricated for comparison (those samples are marked as 0#). Except for C atoms, in order to systematically study the influence of substrate doping on the electronic properties of EG, conventional impurities N, Al, and group two elements O were also injected into SiC and then EGs were fabricated no those substrates. In addition, the existing n-type 4H-SiC homogeneous epitaxy samples were directly taken as N-doped substrates for a comparative study on the effects of injection and in-situ doped substrates on the properties of EG. All EGs were fabricated with two continuous procedures: SiC substrate pretreatment and EG growth. More details 4

are discussed in section I of supplementary information. The characterization of all samples is described in the supplementary information: II.

2.2 The calculation of work function

Synchrotron radiation UPS spectroscopy is very sensitive to the material surface and could intuitively response the slight change of surface electronic structure. In this study, synchrotron radiation UPS spectroscopy was used to examine the electric properties of EG on doped SiC. The cutoff edge of secondary electron is determined by the tangent method and the work function is obtained via the following equation[11, 12]: ϕ = hν − (

−

)

(1-1)

where，ϕ is the work function, hν is the excitation energy, the photon energy used here is 40 eV, is the Fermi level. All tests take Fermi level as zero point,

= 0.

corresponding to the cutoff edge of secondary electron spectra.

3. Results and discussion

3.1 The electronic properties of EG on SiC doped with C ions

3.1.1 Effect of injected C doses on the electronic properties of EG on those doped SiC.

5

is the energy

Figure 1 the C 1s core level spectra of EG on SiC (a) without doping, doped with (b) 5 × 10 cm-2 C ions, (c) 5 × 10 cm-2 C ions.

As discussed in supplementary information: Ⅷ, The C 1s core level spectra of 0# samples (EG on high purity SiC) can be divided into three sub-peaks shown in figure 1 (a): C-Si around 282.8 eV related to 6√3 stucture[13, 14], sp2 around 284.6 eV originating from C-sp2 hybrid components of graphene[15], C-O-C around 286.55 eV related to C-sp3 hybid[16]. the appearance of C-O-C band is caused by the formation of sp3 hybridization at the interface between graphene and SiC, and, the structure of this interface is very similar to that of diamond sp3 hybrid, implying EG strongly bond to the substrate [17]. Based on the C 1s core level spectra, it can be speculated that those samples are composed of !6√3 × 6√3"R30 structure of SiC and their overlay graphene, and, O combination with the interface counteract the interaction between graphene and SiC substrate[18]. But, the C 1s core level of EG on doped SiC is different from the one on pristine SiC. As shown in figure 1 (b) for the C 1s core level

of EG on SiC doped with 5 × 10 cm-2 C, the sp2 band is very close to C-Si 6

band, indicating the degree of order and thickness of 1# samples increase compared with 0# samples. That is, for the same epitaxy growth, more C atoms are accumulated on the surface of SiC doped with C and orderly and consecutive graphene easily forms on those substrates. HO-C=O band which represent sp3 hybridization first appears in this spectrum, indicating the binding process and interface structure of EG and doped SiC have changed compared with that on purity SiC. The intensity of sp2 and C-Si bands in C1s spectrum shown in figure 1 (c) of EG grown on SiC doped with 5 × 10 cm-2 C is significantly increased compared to 1# samples, also implying in the same growth window, high-quality EG is easier formed on doped substrates[19]. The differences of C-O-C and HO-C=O in figure 1(b) and (c) imply the binding mode and interface structure of EG and doped SiC is different as the variation of doped C doses. The most distinct difference between the C 1s core Level spectra of EG grown on SiC doped with various C doses is in the intensity of sp2 bands. It can be seen that a higher quality EG (sp2 component is stronger) is obtained on SiC doped with slight dose 5 × 10 cm-2 C. Although C atoms have the same spatial distribution in SiC, when they were injected with the same energy, the rebounded bulk Si atoms will also increase by the same order of magnitude when injected C ions increase to a larger dose. Therefore, more Si atoms need to escape during graphene growth on SiC doped with high dose C ions, which results in a relatively small sp2 band for EG on SiC doped with larger dose C. As shown in figure 1, the C 1s sp2 band of EG on doped SiC is much stronger than that of purity SiC. It can be concluded that injecting C into SiC can reduce the threshold temperature of Si sublimation, and injected C would serve as an external carbon source which is beneficial to EG growth. the C 1s sp2 peak positions of those three sets samples:

7

284.63±0.01 eV (0#), 284.95±0.01 eV (1#) 285.07±0.01 eV (4#). This sp2 peak position difference is caused by negative charge transfer from SiC to EG; Most of the transferred charges stay close to the buffer layer, which makes EG n-type doping; With the increase of charge transfer, the sp2 peak moves to high binding energy[20-23]. This further demonstrates that the bonding of EG and doped SiC is very different from that on pristine substrates, but the bonding modes of graphene and substrates doped with different C doses of C do not change much.

Figure 2 the UPS spectra of EG on primary SiC, SiC doped with 5 × 10 cm-2 and 5 × 10 cm-2 C ions, (a) valence band spectrum; (b) Secondary electron spectroscopy.

Valence band spectra of 0#, 1# and 4# samples are shown in figure 2 (a). In addition to the weak Si 3p and C 1s bands observed in the high binding energy range of 8.5-17 eV, σ band originated from the hexagonal carbon ring structure can also be observed around 7 eV in the spectra of each sample 8

sets, implying hexagonal carbon ring structure has formed on SiC[21, 24]. π peak derived from the interlayer of hexagonal C- structure can also be observed around 8.0 eV, indicating EG has been formed on SiC[25, 26]. The σ and π peaks dominate the spectrum lines of each sample, indicating those EG in a high degree of order[26, 27]. As seen in figure 2 (a), the σ and π peaks generally move towards low binding energy for samples grown on doped SiC, compared to the one on pristine substrates implying that EG on doped SiC is of higher quality[28]. This quality improvement is induced by the significant decrease of EG growth threshold temperature due to the ion implantation and external carbon source provided by injected C atoms which participate in the formation of EG during the growth window[29]. However, σ and π peaks moving to low binding energy should be attributed to the large change of bonding between EG and doped SiC[30]. Just as indicated by XPS C 1s spectra in figure 1, injecting various dose of C into the SiC make the binding between EG and those SiC change which result in the order of EG on SiC doped with high-dose C is not as high as that on SiC doped with low-dose C. As a result, a large number of transfer charges cannot reach the EG, then, σ and π peaks move slightly toward the high binding energy for EG on SiC doped with a high dose of C[26].

The cut-off positions of secondary electrons spectroscopy of 0#, 1# and 4# samples were determined by the tangent method to be 36.325 eV, 35.157 eV, and 35.959 eV, respectively, as shown in figure 2 (b). The work functions of 0#, 1# and 4# samples can be calculated by the formula (1-1) as follows: 3.675 eV, 4.843 eV, and 4.041 eV, showing a trend: increase first and then decrease. It is known that for materials adhering to lower active substrates, the work function is extremely sensitive to the 9

charge transfer at the interface. If the electrons are transferred from the substrates to the upper cover, the vacuum energy level of the sample will move up, that is, the work function will increase [31-33]. EG on SiC doped with 5 × 10 cm-2 C receives a large number of electrons transferred from the substrates, thus dipoles are formed at the interface, resulting in the increase of vacuum energy level and increase of the work function, which is consistent with the results of XPS C 1s and valence band spectra. Compared with EG on SiC doped with low dose C, EG on SiC doped with large dose C has a slight small work function, as a result of poor order of EG on SiC doped with high dose C.

Figure 3 the Si 2p core level spectra of EG on SiC (a) without doping, doped with (b) 5 × 10 cm-2 C ions, (c) 5 × 10 cm-2 C ions.

According to supplementary information: Ⅷ, the Si 2p core level spectra of 0# samples can be divided into three sub-peaks shown in figure 3 (a): Si-adatoms around 99.8 eV related to adsorbed Si atoms on 6√3 stucture[13], B located around 101.4 eV originating from 6√3 bulk structure and S2 around 103.0 eV related to Si-O compound induced by those samples explosion to air[18, 34-36]. 10

The appearance of the Si-adatoms band indicates the poor order and continuity of 0# samples, as a result, the 6√3 structure can be detected by XPS. The structure of those samples: disorder graphene coving on !6√3 × 6√3"R30 structure and O combined in their interface can be deduced based on the information of Si 2p core level spectra. Si 2p profiles of EG on C doped SiC are completely different from 0# samples. Figure 3 (b) shows that the Si-adatoms which represent the adsorbed Si atoms disappear, the intensity of bulk B band decreases greatly and there is no strong Si-O band at high binding energy of 103.10 eV in Si 2p spectra of 1# samples, indicating that the adsorbed atoms sublimate from the samples and a compact and orderly EG is formed on those substrates. A Si-cluster band at 100.95 eV appears in the Si 2p core level of 1# samples, as shown in figure 3 (b). This band is supposed to be associated with Si clusters which come from the C-Si layer and reach the substrate surface but has not yet escaped from the substrate surface[26]. This Si-cluster band is very intensity, indicating the Si atoms in !6√3 × 6√3"R30 structure and top C-Si layer are sublimated in large quantities but most of them have not yet escaped from the surface of those samples. This accumulation of Si clusters is likely to be responsible for the uplift and fold on the surface of EG reported in literature[5, 37-39]. That is, under the same growth process, the sublimation of Si from doped SiC is easier compared to purity one, thus more order and complete EG tend to form on doped SiC. In addition to the Si-cluster, a Si4C4-xO band related to the oxides of C and Si is also present in this spectrum, which is caused by the exposure to air[12, 18]. Table 2 lists the main bands eigenvalues of the Si 2p core level spectra of 1# and 4# samples. the eigenvalues of Si4C4-xO bands for those two samples do not change much, indicating that for SiC doped with different doses, the

11

binding between EG and those substrates has few differences. The most dissimilarity of Si 2p core levels for 1# and 4# samples are the intensity of Si-cluster band and bulk B band. The intensity of Si 2p Si-cluster band of 4# samples is greatly enhanced, compared to that of 1# samples. This is because, although C ions have the same spatial distribution in SiC when they were injected at the same energy, the rebounded bulk Si atoms will also increase by the same order of magnitude when C ions are injected at larger doses. Therefore, during the same growth window, more Si atoms stay at the surface of SiC doped with high dose C, which results in Si cluster stacking and a relatively stronger Si-cluster band. Table 2 the main component eigenvalues of the Si 2p core level of 1# and 4# samples. SiC(5 × 10 cm-2 C)

SiC(5 × 10 cm-2 C )

Component BE* (eV)

GW† (eV)

Area‡ (P)

BE(eV)

GW(eV)

Area(P)

Si-cluster

100.95

0.94

18959.3

100.96

0.96

22443.22

B

101.48

0.94

9672.4

101.49

0.96

11449.77

102.38

1.97

4680.03

102.62

1.94

4603.27

103.11

1.97

2387.59

103.15

1.94

2348.43

Si4C4-xO

*

Binding Energy(BE), † half-width of subpeak (Gauss, GW), ‡ Intensity of subpeak (Area).

3.1.2 Effect of injection energy on the electronic properties of EG on those doped SiC

12

As shown in figure S9, the C 1s core levels spectra of 1#, 2# and 3# samples contain four sub-peaks: The C-Si band located about 282.8 eV and related to 6√3 structure[13, 14]; the C-sp2 band around 284.6 eV derived from graphene[15]; the C-O-C band related to C-sp3 around 286.55 eV [16]; and the HO-C=O band located at high binding energy 288.88 eV. For 1# samples, the intensity of the sp2 band is very close to that of C-Si band, indicating more order EG has formed on SiC doped with 5 × 10

cm-2 C atoms and 30 keV. Compared with EG on primary SiC, the appearance of HO-C=O

is the result of the new binding structure between graphene and doped SiC[18]. For EG grown on SiC doped with different injection energy, the C-O-C and HO-C=O bands are quite different, indicating the binding structure of EG and those substrates are distinct. Table 3 lists the detail information of C 1s core level spectra of 1#, 2# and 3# samples. The intensity of C-Si band for the three samples is very different. The C-Si band of 2# samples has the largest intensity. When SiC doped with injection energy 35 keV, although the C injection and the bounced Si and C atoms would change the C-Si layer, these changes are not enough to destroy the topmost C-Si layer on a large scale; as a result, the Si sublimation rate is still dominated by the C-Si layer. But, during the same growth window, a large number of rebounded Si atoms need to run out of those SiC surface which makes the orderliness of 2# samples is not as good as that of 1# samples, thus the C-Si band of 2# samples is stronger than that of 1# samples. When the injection was performed at 40 keV, a great number of Si and C atoms are bounced back to the near surface of SiC. In this case, the structure of the topmost C-Si layer is greatly changed by high energy injection, which makes the sublimation of Si much easier; in addition, more rebound C atoms and injected C ions serve as a second carbon

13

source for graphene growth; as a result, higher quality EG formed on SiC doped with 40 keV during the same growth window. The C 1s core level of those samples have lower C-Si band and higher sp2 band. The sp2 intensity in C 1s core level of EG decreased after the first increasing as their substrates were doped with injection energy varying from 30 keV to 40 keV, under the same C dose, shown in table 3. Just as mentioned above, as the injection energy changed, the EG presents a texture and orderness from high to low then to high. The positions of C 1s sp2 bands of 1#, 2# and 3# are 284.95 (+0.01 eV), 285.14 (+0.01 eV) and 284.66 (+0.01 eV), respectively. This shift is caused by the transfer of negative charges from SiC to EG. With the increase of charge transfer, the sp2 band moves to high binding energy but to low binding energy with the decrease of charge transfer[13, 18, 19, 40, 41]. The position of C1s sp2 of 3# samples is basically the same as that of EG on the purity SiC, shown in figure 1 (a) (284.63 eV), showing that the n-type doping degree of EG is very small in this case, which is consistent with the change of the bonding structure between EG and various doped substrates.

Table 3 the main component eigenvalues of the Si 2p core level of 1#, 2# and 3# samples. SiC(5 × 10 cm-2 C、30 keV) Feature C-Si

sp2

C-O-C

HO-C=O

BE*(eV)

283.19

284.95

286.35

288.91

GW†(eV)

0.93

1.42

1.39

1.17

Area‡(P)

33617.12

42854.41

11659.54

7805.85

14

SiC(5 × 10 cm-2 C、35 keV) BE(eV)

283.26

285.14

285.76

-

GW(eV)

0.97

1.35

3.37

-

Area(P)

41459.54

37750.46

6354.78

-

SiC(5 × 10 cm-2 C、40 keV) BE(eV)

282.62

284.66

286.05

288.59

GW(eV)

0.94

1.31

1.31

1.05

Area(P)

34483.6

39896.85

10398.98

6079.25

*

Binding Energy(BE), † half-width of subpeak (Gauss, GW), ‡ Intensity of subpeak (Area).

The dominant states σ originating from the hexagonal carbon ring structure and π which derive from the interlayer structure of hexagonal carbon layers around 8.0 eV can also be observed in valence band spectra of 1#, 2# and 3# samples, shown in figure 4 (a), conforming that ordered graphene has formed on SiC[21, 24-27]. As seen in figure 4 (a), the σ and π bands of EG move towards high binding energy when the energy with which their substrates were doped increases from 30 keV to 35 keV, indicating that the amount of electron transferred to the EG decreases when the substrates they grew on was doped with energy 35 keV which is caused by the poor order of EG on those substrates[28, 40]; Just as shown by XPS results in table 3, a large number of transformed electrons stay close to the buffer layer, but combination position of the buffer layer and EG traps relatively few electrons. As a result, the σ and π bands of 2# samples move slightly towards high binding energy[26]. But, the σ and π bands of 3# samples whose substrates doped with energy 40keV move 15

slightly towards lower binding energy relative to 2# samples. In this case, a larger number of rebounded and injected C atoms service as second carbon sources for graphene growth [29]; and, high energy injection and a larger dose rebounded atoms greatly broke the structure of C-Si layer. As a result, the σ and π bands of 3# samples move slightly towards lower binding energy[30], which is consistent with the previous XPS C 1s results.

The cut-off positions of secondary electrons spectroscopy of 1#, 2# and 3# samples were determined by the tangent method to be 35.157 eV、36.131 eV、35.515 eV, respectively, shown in figure 4 (b). Also, their work functions can be calculated by formula (1-1) as follows: 4.843 eV、3.869 eV、 4.485 eV, respectively. For those samples, the work function decreases first and then increase. As discussed above, if the electrons are transferred from SiC substrates to EG, the work function will increase[31-33]. The orders of 2# samples are not as good as that of 1# samples. Most of transferred negative charges stay close to the buffer layer and the effective charges transfer to EG is low. As a result, EGs on SiC doped with energy 35 keV have a relatively small work function. Compared to 1# and 2# samples, the combination mode of EG and SiC doped with energy 40 keV has undergone tremendous changes. EG on those substrates receives vast electrons from substrates and dipoles forms at the interface, which results in the increase of the vacuum energy level, i.e. the work function increase, consistent with the results of XPS C 1s core level and valence band spectrum.

16

Figure 4 the UPS spectra of EG on SiC doped with different injection energy. (a) valence band spectrum; (b) Secondary electron spectroscopy. Bulk B band around 101.4 eV related to 6√3 structure, Si-cluster band around 100.95 eV supposed to be associated with Si clusters which come from the C-Si layer and reach the substrate surface but has not yet escaped from the sample surface[26] and Si4C4-xO band associated with the mixed oxides of C and Si[12, 18] appear in the Si 2p core level spectra of 1#, 2# and 3# samples as shown in figure S10. The Si-adatoms band[13] around 99.8 eV as shown in figure 3 (a), does not appear in figure S10, indicating there are few Si atoms absorbed on the samples, and compact and ordered EGs are formed on those substrates. For those three sample sets, the intensity of B band is much weaker than that of the Si-cluster band indicating the Si atoms of the !6√3 × 6√3"R30 structure and C-Si layer 17

sublimate in large quantities and deposit on topmost of the buffer layer but has not yet escaped from the sample surface. Based on those spectra, it can be speculated that those samples have a structure: ordered EG covers on !6√3 × 6√3"R30 structure and owing to the special binding mode of EG and doped SiC, O and Si combine with the buffer layer between EG and SiC. Table 4 lists the main band eigenvalues of Si 2p core level spectra of 1#, 2# and 3# samples. Except for the intensity, the Si-cluster and B bands for 1# and 2# samples have no obvious difference. The changes of C-Si layer caused by the C injection and rebounded Si and C atoms are not enough to destroy the original C-Si structure on a large scale, so, the Si sublimation during EG growth still dominated by the C-Si layer; and a large number of rebounded Si atoms need to run out of the sample surface. As a result, no enough C atoms participate in fabricating ordered EG which make the Si 2p spectra of 2# samples present strong Si-cluster and weak B bands. Compared to 1# samples, Si4C4-xO band in the Si 2p core level of the EG on SiC doped with energy 35 keV shifts about 0.5 eV to high binding energy, implying the bonding of EG and SiC doped with different energy are quite different. Compared to 1# and 2# samples, all bands in Si 2p core level of EG on SiC doped with energy 40 keV move towards low binding energy. In this case, injection and rebounded atoms make the near-surface of SiC change a lot, which results in a large change of binding between EG and SiC. As a result, the charge transfer mode has changed obviously, leading to the Si 2p bands move towards low bonding energy[42, 43].

Table 4 the main component eigenvalues of the Si 2p core level of 1# ,2# and 4# samples.

Feature

SiC(5 × 10 cm-2 C、30 keV)

18

Si-cluster

B

Si4C4-xO

BE*(eV)

100.95

101.48

102.38

103.11

GW†(eV)

0.94

0.94

1.97

197

Area‡(P)

18959.30

9672.40

4680.03

2387.59

SiC(5 × 10 cm-2 C、35 keV) BE(eV)

101.00

101.53

102.87

103.40

GW(eV)

0.97

0.97

1.85

1.85

Area(P)

23342.18

11908.40

4020.32

2051.03

SiC(5 × 10 cm-2 C、40 keV)

*

BE(eV)

100.33

100.98

102.27

102.80

GW(eV)

0.90

0.90

1.64

1.64

Area(P)

20239.43

10325.48

3392.75

1730.87

Binding Energy(BE), † half-width of subpeak (Gauss, GW), ‡ Intensity of subpeak (Area).

3.1.3 Effect of C implantation in substrates on EG with the same thickness

19

Figure 5 Raman spectra of EG with the same thickness on purity SiC and substrate doped with C ions.

As shown above, under the same growth process, high quality EG can be prepared on C doped SiC. When fabricating EG on SiC doped with C ions, the growth threshold temperature decreases and doped C ions serve as a second carbon source accelerating the formation of EG. As discussed in supplementary information: Ⅴ, compared with high purity SiC substrate, it is easier to fabricate EG on doped SiC under the same growth process. Compared with the growth on pristine SiC, for SiC doped with C ions, the EG growth threshold temperature has decreased by around 105℃.

Raman spectra of EGs with the same thickness on purity and doped SiC have approximately the same profile, shown in figure 5: strong 2D and G peaks and no obvious D peak, indicating that high-quality EGs are grown on those two substrates[44-46]. The integral strength ratio of the 2D and 20

G peaks (I2D/IG) in the Raman spectra of the two sets EGs is less than 1 and the HWHM of the 2D peak in those spectra are 30 cm-1, indicating that monolayer graphene has formed on those two sets substrates. The difference of those Raman spectra is the peak position of G and 2D peak. The Raman G and 2D peaks of EG on intrinsic SiC are located at 1581.8 cm-1 and 2683.6 cm-1 respectively, while the Raman G and 2D peaks on C doped SiC are located at 1592.5 cm-1 and 2693.2 cm-1 respectively, shown in figure 5 (b) and (c). As discussed in supplementary information: Ⅵ and Ⅶ, for continuous and uniform EG on SiC, the substrate surface doping would induce the Raman modes change of those EG. Hence, the blue shifts of Raman G and 2D peaks in figure 5 are caused by the increase of charge transfer arising from the binding change of EG and doped substrates[47-50] which is consistent with the theoretical calculations[51, 52].

21

Figure 6 the UPS spectra of EG with the same thickness on purity SiC and substrate doped with C ions. (a) valence band spectrum; (b) Secondary electron spectroscopy.

The UPS of EGs of the same thickness on purity and doped SiC are shown in figure 6. The valence band spectra of the two kinds of EGs basically have the same profile, except that the π band around 8 eV is slightly shifted to low binding energy due to the change of the bonding between EG and SiC resulting from the SiC doping. As mentioned above, the work function of EG is sensitive to the charge transfer. The electrons transfer from substrates to the cover layer will lead to the moving up of vacuum energy level[31-33]. The work function of those two set EGs can also be extracted from Figure 6 (b). The work function of EG on doped SiC is larger than that on purity SiC, that is, a large number of electrons have transferred from doped SiC to EG.

3.2 Discussion on the electronic properties of EG grown on N, Al, O doped SiC

Figure S11 shows the C 1s core level spectra and their overlapping peaks of EGs grown on SiC doped with different doses of N, Al and O. Those C 1s spectra prove that EGs have formed on all substrates, but various substrates doping make the binding structure of EGs and doped SiC difference, which result in the variation of the sub-peak position and HWHM in the C 1s spectra of each samples. For the C 1s cove level of EG grown on n-type SiC homogeneous layer, the sp2 band is stronger than C-Si band indicating that the in-situ doping during homoepitaxial growth greatly preserves the hexagonal structure of 4H-SiC which is beneficial to the formation of ordered EG. But for the C 1s spectra of EG grown on other doped SiC, the sp2 band is weaker than C-Si band, as shown in Table 5. Except EG grown on SiC homogeneous layer, HO-C=O do not appear in the C 1s 22

core level spectra of other samples, again indicating the combination of EG with intrinsic SiC is very different from that of EG with doped SiC. The appearance of HO-C=O sub-peak is regarded as the sign of substrates in-situ doping, as shown in figure S11 (a)[53]. The intensity ratio of those XPS sp2 and C-Si sub-peaks shows that monolayer EG has grown on those substrates[54, 55]. Comparing table 3 and 5, it can be seen that injecting Ⅲ,Ⅴ,Ⅵ group atoms in SiC would induce the sub-peaks in C 1s cove level of EGs grown on those substrates further moving towards low binding energy, although the position of Si-C, sp2, and C-O-C peaks in C 1s core level of EGs on SiC doped with of 5 × 10 $ cm-3 N only have slight shift relative to that of EG on SiC doped with of 5 × 10 cm-2 C and 40 keV. It could be concluded doping SiC with different groups atoms make EGs on those substrates have various electronic properties. This further bands shift to low binding energy is induced by the great changes of binding between EG and SiC arising from SiC doped with Ⅲ,Ⅴ,Ⅵ group atoms which results in the p-type doping of EG[40, 56, 57]. Moreover, the charge transfer caused by these doping is much more than that caused by C doping.

Table 5 the main component eigenvalues of the C 1s core level of EG on SiC doped with (a) 5 × 10 % cm-3 N, (b) 5 × 10 cm-2 N, (c) 5 × 10 $ cm-2 N, (d) 5 × 10 cm-2 O, (e) 5 × 10 & cm-3 Al samples. N 1 × 10 % cm-3 Feature

BE(eV)

C-Si

sp2

C-O-C

HO-C=O

281.84

283.60

285.31

287.40

23

GW(eV)

0.83

1.38

1.75

1.89

Area(P)

2761.71

77624.01

15970.38

4445.23

N 1 × 10 cm-3 BE(eV)

282.11

284.20

285.79

-

GW(eV)

0.94

1.31

1.45

-

Area(P)

53182.88

17505.13

3588.47

-

N 1 × 10 $ cm-3 BE(eV)

282.30

284.60

286.15

-

GW(eV)

0.86

1.26

1.31

-

Area(P)

52378.59

11916.34

3164.92

-

O 1 × 10 cm-3 BE(eV)

282.19

284.28

285.81

-

GW(eV)

0.85

1.27

1.45

-

Area(P)

557447.2

13436.09

3053.97

-

Al 1 × 10 & cm-3 BE(eV)

281.82

284.07

285.12

-

GW(eV)

0.82

1.09

2.23

-

46914.01

25430.55

6772.82

-

*

Binding Energy(BE), † half-width of subpeak (Gauss, GW), ‡ Intensity of subpeak (Area).

3.3 The changes of the valence band and work function with different SiC doping 24

As seen in figure 7, compared to EG grown on purity SiC, the C 1s sp2 peaks of EG on SiC doped with N, Al and O move towards low binding energy in general: The C 1s sp2 peak of EG on SiC doped with 1017 cm-3 N has a shift up to 1.03 eV, and the C 1s sp2 peak of EG on SiC doped with 1015 cm-2 N has a small shift 0.3 eV, but for EG on SiC doped with Al and O, the displacements of C1s sp2 peak are 0.56 eV and 0.35 eV. As mentioned in sectionⅡ, compared with EG on purity SiC (figure 1 (a)), the sub-peaks in C 1s core level of samples discussed in section Ⅱ move to low binding energy in varying degrees which is caused by the p-type doping of EG related to the unique binding structure of EG and SiC doped with non-homogeneous elements. Based on sectionⅠandⅡ, it is known that the substrate surface doping will cause the changes of binding structure between EG and substrate, which will make EG doped; the doping type of EG has nothing to do with the type (donor or acceptor) of impurities doped into SiC but is related with the unique change of the binding structure; Further research is needed on these changes of heterostructures. It could be drawn that SiC doped with atoms of group Ⅵ make EG on those doped substrates n-type but SiC doped with other group atoms results in EG on those substrates doped with very few electrons. As shown in figure 7, the HWHM of C 1s sp2 peak for EG on doped SiC is generally smaller than that of EG on purity SiC; for EGs on SiC doped with C, the HWHM of C1s sp2 peaks has a decreasing trend with the increase of injection dose and energy; for EGs on SiC doped with other groups atoms, such as N, the HWHM of sp2 peak decreases with the decrease of doping dose. The specific reasons for those phenomena need to be further studied. The HWHM of sp2 peak of EG is broadened if the substrate was doped with a low dose O. This broaden of HWHM can be attributed to the presence of O atom.

25

Figure 7 The position and HWHM of sp2 band in XPS C 1s core level spectra of EG grown on pristine and doped SiC. In the figure, pristine represents EG grown on undoped SiC. C (30/1014) represents EG grown on SiC doped with 1014 cm-2 C ions and 30 keV. By analogy, N (epi/1017) represents EG grown on epitaxial 4H-SiC doped with 1017 cm-3 N. the similarity representation was applied to other samples.

26

Figure 8 UPS spectra of EG on the various substrate, the representation of samples in this diagram is consistent with Figure 7.

As shown in figure 8 (a), compared to EG on purity SiC, the σ and π bands in valence band spectra of EGs on SiC doped with C move to low binding energy, which results from the improvement of EG quality and the unique structure between EG and SiC induced by substrate doping with C[29, 30]. The order of EG on SiC doped with high dose C is not as high as that on substrates injected with low dose C, therefore σ and π bands in UPS spectra of EG on SiC doped with high dose C are at higher binding energy compared the one on SiC doped with lower dose C[26]. When the C injection energy increases from 30 keV to 35 keV, the σ and π bands of EGs on those doped substrates move 27

to high binding energy which indicates that the amount of electron transferred to EG from SiC doped with 35 keV and the order degree of those EG decreases compared with the one on SiC doped at 30 keV[26, 28, 40]. Instead, when the injection energy is increased to 40 keV, a large number of rebounded C atoms and injected C ions as second carbon source participate in the formation of EG [29], in addition, high energy injection and large dose of C and Si rebounded atoms result in the great change of C-Si layer structure near substrate surface, both of them make the σ and π bands in the valence band spectra of EG on those substrates move to the low binding energy direction[30].

The position of σ band in valence band spectrum of EG on SiC doped with N is higher than that of EG on purity substrates on the whole, but, with the decrease of injection concentration, the σ band position of those samples gradually approaches to that of EG on purity SiC as shown in figure 8(a) which is in good agreement with the XPS results in figure S11. That is, with the decrease of concentration of N injected into SiC, the p-type carrier’s concentration of EG on those doped SiC decreases correspondingly. But, σ band in the valence band spectrum of EG on SiC doped with Al shifts to a much smaller direction than that of EG on purity SiC, although the XPS results show that the EG is p-doped. The specific mechanism of this contrast is still unclear and needs further study. when the doped atoms become 1014 cm-2 O, the σ band in valence band spectrum of EG on those substrates shifts to a higher binding energy than that on the SiC without doping. Based on the above analysis, it can be preliminarily concluded that the inverse change trend of XPS and valence band spectrum of EG on SiC doped with Al is due to the fact that Al is the acceptor atom. That is,

28

although EG on SiC doped with non-Ⅵ group exhibits p-type doping, the specific mechanism on p-type doping of EG on SiC doped with donor or acceptor atoms is different.

In order to characterize the effect of substrate doping on the bandgap of EG, the valence band spectra in figure 8 (a) are enlarged in the region between -0.2 eV and 2 eV, shown in figure 8 (b). The distance from the valence band top to the Fermi level (EF-EV) of all samples shown in figure 9 was obtained using the tangent method: starting from 0 eV point (Fermi level), the beginning of the first peak is found to make a tangent line. Then, EF-EV is determined by the intersection position of this tangent line and the baseline of the spectrum[58-60]. It can be seen that the EG on SiC doped with 5 × 10 cm-2 C and injection energy 30 keV has minimum EF-EV, but the EF-EV increases in varying degrees for EG on substrate doped with other increased C does or injection energy. The EF-EV of EG on SiC doped with 5 × 10



cm-2 C, 30 keV and 5 × 10 cm-2 C, 35 keV is larger

than that of EG on purity SiC. The variation of EF-EV of EGs on SiC doped with C is consistent with results given by XPS C 1s and valence band spectra – The higher the electron concentration in EG is, the smaller the EF-EV becomes. As can be seen in figure 9, EF-EV of EG on SiC doped with N decreases as the concentration of N doped into SiC decreases. This variation tendency of EF-EV is consistent with the analysis results of XPS and valence band spectra shown in section Ⅱ: for p-type EG, when the electron concentration in those EG increases, EF-EV of those EG decreases. The EG on SiC doped with Al exhibits strongly p-type property based on XPS results shown in figure S11, according to the above analysis, the EF-EV of those samples should be larger than the one of EG on purity SiC. But figure 9 shows that the EF-EV of EG on SiC doped with Al is quite small, which is 29

consistent with the changing trend of σ band in valence band spectrum shown in Figure 8. Acceptor Al results in this smaller EF-EV. Also, EG on SiC doped with O has a lager EF-EV compared with the one on purity SiC.

The cut-off edges of secondary electrons spectra for EG on each substrate (from bottom to top in figure 8 (c)) determined by tangent method are at 36.325 eV, 35.157 eV, 35.959 eV, 36.131 eV, 35.515 eV, 36.614 eV, 36.588 eV, 36.528 eV, 35.471 eV and 36.432 eV, respectively and the work functions (denoted by WG) of each samples can be calculated by formula (1-1) as follows: 3.675 eV, 4.843 eV, 4.041 eV, 3.869 eV, 4.485 eV, 3.386 eV, 3.412 eV, 3.472 eV, 4.529 eV and 3.568 eV, shown in figure 9. It is known that for some materials adhering to lower active substrates, the work function of those materials is extremely sensitive to the charge transfer at the interface. If the electrons are transferred from the substrates to the upper cover, the vacuum level of those samples will move up, that is, the work function will increase[31-33]. Therefore, the variation of these work functions shown in figure 9 can be explained by the degree of charge transfer, which is consistent with the electron transfer phenomena revealed by XPS and valence band spectra.

The distance from the valence band top to the vacuum level (WG+Ep) is easily obtained based on the data of EF-EV(distance from the valence band top to the Fermi level) and WG (work function of epitaxial graphene), WG+Ep = EF-EV + WG, shown in figure 9. The EF-EV obtained from valence band spectra could clearly distinguish the distance between the valence band top and the Fermi level of each sample. As discussed in supplementary information: Ⅸ, the bandgap width of epitaxial graphene can be determined using WG+Ep obtained from valence band spectra, the bandgap 30

width=WG+WP-χ, where χ is the affinity potential of epitaxial graphene. It is known that EG on pristine SiC substrate has a bandgap 0.26 eV. Therefore, it is reasonable to qualitatively illustrate the relationship between of substrate doping (under various conditions) and the bandgap of EGs on those doped substrates by comparing WG+Wp obtained from the UPS spectra of EG on each doped substrate to that of EG on pristine SiC. It can be seen from figure 9 that except the EG on SiC doped with low dose O, the bandgap of EG on other doped SiC has been broadened. However, the specific broaden mechanism of EGs on doped SiC is different due to the different substrate doping, which needs further study.

In summary, when doping different atoms into SiC, the bandgap and work function of EG grown on these substrates are obviously different. The electronegativity of atoms with different atomic number varies greatly, and the volume of atoms with various number is also significantly different compared with each other. Therefore, the heterostructures between EG and SiC is of obvious differences due to their substrates doped under different condition. This structure difference induces variation of charge transfer mode and magnitude which manifest as that EG on SiC doped under different condition has different bandgap width and work function.

31

Figure 9 The distance from the valence band top to the Fermi level EF-EF; the work function WG; and the distance from the valence band top to the vacuum level WG+Ep, where Ep is a hole Fermi potential for each sample. The representation of samples in this diagram is consistent with Figure 7.

4. Conclusion

In this work, the effect of substrates doping on the bandgap of EG on those doped SiC were studied in detail. The bandgap of EG grown on 4H-SiC(0001) can be widened via a novel charge transfer mechanism by modifying the buffer layer and surface of SiC with substrate doping before epitaxial growth. Both donor-acceptor and bulk atoms injected in the SiC make the bandgap of EG on those substrates broaden. When doping different atoms into SiC, the bandgap and work function of EG grown on these substrates are obviously different. The electronegativity of atoms with different 32

atomic number varies greatly, and the volume of atoms with various number is also significantly different compared with each other. Therefore, the heterostructures between EG and SiC is of obvious differences due to their substrates doped under different condition. This structure difference induces variation of charge transfer mode and magnitude which manifest as that EG on SiC doped under different condition has different bandgap width and work function. This novel surface charge transfer doping mechanism via substrates doping will provide a simple and effective approach to nondestructively broad the bandgap of epitaxial graphene for future electronics application.

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website.

Materials preparation, methods, and supporting figures S1-S13 (PDF)

Author Contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Jialin Liua, Wentao Doub, Kaili Maoc, Lixin Guod, Laiyuan Chonga, Jichao Hue, Hao Yuana,*, Yanjing Hea, Hui Guoa,*, Yuming Zhanga contributed equally.

Notes The authors declare no competing financial interest.

Acknowledgment

33

This work was supported by grants from the Fundamental Research Funds for the Central Universities [Grant No. JB181109] and National Natural Science Foundation of China [Grant No. 61575153 and 61675191).

Abbreviations

SiC, Silicon Carbide; EG Epitaxial Graphene on SiC; GFFs, graphene-based Field Effect Transistors; ICs, Integrated Circuits; GNRs, Graphene Nanoribbons; AFM, Atomic Force Microscope; RMS, Root Mean Square; FWHM, Full Width at Half Maxima; UPS, Ultraviolet Photoelectron Spectroscopy;

SRPES,

Synchrotron

Radiation

Photoelectron

Spectroscopy;

XPS,

X-ray

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