4H–SiC(0001) by Joule heating decomposition in hydrogen

Journal Pre-proof Tuning electronic properties of epitaxial multilayer-graphene/4H–SiC(0001) by Joule heating decomposition in hydrogen Cunzhi Sun, Weiwei Cai, Rongdun Hong, Junkang Wu, Xiaping Chen, Jiafa Cai, Feng Zhang, Zhengyun Wu PII:

S0022-3697(19)30465-2

DOI:

https://doi.org/10.1016/j.jpcs.2019.109224

Reference:

PCS 109224

To appear in:

Journal of Physics and Chemistry of Solids

Received Date: 28 February 2019 Revised Date:

19 September 2019

Accepted Date: 4 October 2019

Please cite this article as: C. Sun, W. Cai, R. Hong, J. Wu, X. Chen, J. Cai, F. Zhang, Z. Wu, Tuning electronic properties of epitaxial multilayer-graphene/4H–SiC(0001) by Joule heating decomposition in hydrogen, Journal of Physics and Chemistry of Solids (2019), doi: https://doi.org/10.1016/ j.jpcs.2019.109224. 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.

Tuning electronic properties of epitaxial Multilayer-Graphene/4H-SiC(0001) by Joule Heating Decomposition in hydrogen Cunzhi Sun1， Weiwei Cai1,2，Rongdun Hong1,2,*，Junkang Wu1, Xiaping Chen1,2, Jiafa Cai1,2, Feng Zhang1,2,**, Zhengyun Wu1,2,3 1

Department of Physics, Xiamen University, Xiamen, 361005, P. R. China

2

Jiujiang Research Institute of Xiamen University, Jiujiang, 332000, P. R. China

3

Fujian Key Laboratory of Semiconductor Materials and Applications, Xiamen, 361005, P. R. China

*Corresponding author. Department of Physics, Xiamen University, Xiamen, 361005, P. R. China **Corresponding author. Department of Physics, Xiamen University, Xiamen, 361005, P. R. China E-mail address: [email protected] (Rongdun Hong), [email protected] (Feng Zhang).

Abstract On the semi-insulating 4H-SiC (0001) surface, hydrogenated multilayers graphene (MLG) were epitaxially prepared by the method of Joule heating decomposition in the hydrogen atmosphere. The structural and chemical characteristics of multilayers graphene have been elaborately analyzed by the X-ray photoelectron and Raman spectroscopies, showing the level of hydrogenation being promoted with the increase of hydrogen pressure. Then, diodes with MLG/4H-SiC contact were fabricated and studied, proving that the Schottky barrier height (SBH) of MLG/4H-SiC junction was enhanced by the hydrogenation. By studying the typical

current-voltage

characteristics, the SBH was observed to be heightened from 0.84 eV to 1.0 eV along with the hydrogen pressure increasing from 10-2 mbar to 102 mbar. Finally, graphene-semiconductor-graphene photodetectors were fabricated, showing peak 1

responsivity as high as~ 0.9 A/W and external quantum efficiency of 345%, under the 324 nm illumination and biased at 3V.

Keywords: graphene, 4H-SiC, hydrogenation

1. Introduction Graphene, a zero band gap semiconductor, comprises of one monolayer of carbon atoms arranged in a two-dimensional (2D) honeycomb lattice [1, 2]. In comparison with other semiconductor materials, graphene exhibits remarkable electronic transport properties which make it a prominent candidate for future micro- and nano-electronic applications and a replacing part of the current Si-based technologies [3-5]. Several approaches have been successfully developed for preparing graphene: mechanical exfoliation of graphite [5], chemical vapor deposition (CVD) [6-8], solid source deposition [9, 10] on transition metals, and epitaxial growth on silicon carbide (SiC) utilizing surface graphitization [11-14]. The epitaxial graphene formed on SiC by surface graphitization has an essential benefit in terms of uniform large-scale growth and combination of two stable materials. It is important to open the bandgap in the graphene for further application. Until now, several schemes have been applied, such as using electric fields [15, 16], doping with hetero atoms [17, 18], chemical functionalization [19, 20, 21], depositing graphene on substrates like SiO2 [22]，SiC [23], and cubic BN (111) [24, 25], adsorption of polar molecules [26], uniaxial strain [27] and electronic confinement [11, 2

28, 29]. Amongst these, it is a promising route to break the symmetry of graphene’s lattice and transform the hybridization state of graphene’s C sp2 into C sp3 (HGC) by hydrogenation, increasing uncertainty in the crystal momentum k, as shown theoretically [30-32] and experimentally [33-36]. On the other hand, the common n-type character of as-grown graphene on SiC (0001) is most plausibly explained by the breaking of buffer layer’s sublattice symmetry owing to the graphene-substrate interaction [37] and resultant charge transfer from the interface density of states associated to the buffer layer and the silicon dangling bonds at the SiC substrate surface. Experiments showed hydrogenation of Si dangling bonds (HSD) at the interface of graphene and SiC can restore the asymmetry of buffer layers and restrict the charge transfer, which lead to shift the Fermi level back from above to the Dirac point and even result graphene in slightly p doping [38-40]. For the HGC, the bandgap of graphene can be tuned by different coverage of hydrogen: for example, fully hydrogenated graphene being referred to as graphane (a semiconductor with 3.5 eV~3.7 eV wide-bandgap) [21], single hydrogen atom in per (4×4) supercell [30] generating gap of about 1.25 eV and half-hydrogenated graphene having a bandgap of 0.43 eV [41]. In the case of HSD, the band gap of graphene could be modulated with depinning of Fermi level: the Fermi level of as-grown graphene shift above the Dirac point of ~0.4 eV for high density of Si dangling bonds at the interface [13], whereas the HSD results in the Fermi level shifting above the Dirac point of ~0.1 eV [39, 42]. The essential experiment for studying the hydrogenated graphene has been finished, such as hydrogen has been shown to be adsorbed on free graphene [43], as well as on 3

supported graphene layers [44, 46]. In this letter, we employed a Joule heating decomposition (JHD) to prepare hydrogenated multi-layer graphene (MLG) on a 4H-SiC samples in hydrogen environment by using the Joule heating to generate the sublimation of Si atoms and the recombination of C atoms. Via carefully analysis of the structural and electronic transport characteristics of the hydrogenated MLG, the diodes based MLG/4H-SiC junction and the graphene-semiconductor-graphene (GSG) photodetectors were fabricated and characterized.

2. Experiment details Three rectangular (25.0 mm×5.0 mm) 4H-SiC samples, which have N-type doped (1.0×1015 cm-3 & ~4.0 µm thick) epitaxial layer (0001) on commercial substrate (~0.02 Ω·cm & ~500.0 µm thick), were used for the MLG growth. After RCA cleaning, the SiC samples were put between two Mo electrodes and applied the DC current (2.7 A), as shown in figure. 1(a). In vacuum (5×10-5 mbar) and different hydrogen pressures (1.0×10-2 mbar, 1.0×10-1 mbar and 1.0×102 mbar), the 4H-SiC samples were heated to 1400℃ for 7 minutes by Joule heating generated from the DC current and the MLGs were grown. The structural and chemical configurations of samples have been characterized by Raman spectra (HORIBA Scientific, λ=633nm) and X-ray photoelectron spectroscopy (XPS) using quantum 2000 Scanning ECSA Microprobe from Philadelphia USA. Then, by employing the semiconductor planar technologies of photolithography and magnetron sputtering, diodes based on MLG/4H-SiC junction along with a Cr/Au 4

(10nm/100nm) pads and GSG structure were fabricated. Then, the electrical and photoelectrical characteristics of the devices have been studied, by utilizing a Keithley 2410 sourcemeter, a Keithley 6514 electrometer, a xenon lamp light source and a monochromator.

3. Results and discussion 3.1. Preparation and hydrogenation of MLG Figure 1(a) depicts Raman spectra of 4H-SiC substrate and the as-grown MLG on it in vacuum. The inset depicts the schematic diagram of JHD equipment, showing that the 4H-SiC sample is connected by two molybdenum (Mo) electrodes on both ends of the sample’s long side. In addition, 2D peak of as-grown graphene sample shows symmetry structure and can be fit well by single Lorentz peak [45], indicating ABA stack structure of MLG.

Figure 1 (a) Raman spectra of 4H-SiC substrate and the MLG prepared at 1400℃ by JHD method in vacuum (5×10-5 mbar). And, schematic diagram (inset) of the JHD system and the single Lorentz fit of 2D peak. (b) AFM mapping of as-grown sample with MLG with 6μm scale bar. (c) Schematic representation of hydrogenation process: part of H atoms would pin to Si dangling 5

bonds at the interface and part carbons of MLG would be in the sp3 hybridization.

It is obvious that, for the MLG, several peaks in the Raman result can be observed: the defect-induced D band at peak frequency of ~1352cm-1, the in-plane vibrational G band at peak frequency of ~1582cm-1, the two-phonon 2D band at peak frequency of ~2712cm-1. The 2D curve with a full width at half maximum (FWHM) of 66 cm-1 could be indicative of multilayer graphene [30]. And, two Raman bands, relating to the 4H-SiC with relatively low intensity (peaks at 1522 cm-1 and 1710 cm-1), were observed as well for both samples. Figure 1(b) depicts the AFM mapping of the as-grown sample’s surface where part of MLG has been etched completely by O2 plasma to form a step. And it comes out to be 29±8 layers of MLG, calculating by the step height and the single graphene layer thickness (0.34 nm). To study the influence of hydrogen pressure on the structural and electrical characteristics of MLG, three samples have been prepared in different hydrogen pressures (1×10-2 mbar, 1×10-1 mbar and 1×102 mbar). In this situation, 4H-SiC samples with decomposed MLG would be exposed to molecular hydrogen which was cracked alongside 4H-SiC surface [44]. Part of the hydrogen atoms may be pinned to C sp3 bonds, whereas part of the hydrogen may pass the structural defects and be pinned to Si dangling bonds on 4H-SiC’s surface, as shown in figure 1(c). 3.2. XPS characterization Figure 2 (a)-(d) shows the C1s XPS spectra of MLG samples prepared in hydrogen atmosphere with different pressures. All the XPS spectra of C1s have been background corrected and fitted by Lorentzian-Gaussian function into four peaks, including sp2 6

hybridized C atoms in graphene position at 284.5 eV [47], disordered carbon at 284.8 eV [48], sp3 hybridized carbon atom with H atom at 285 eV [47] and the C-Si (at 285.6 eV) in the interface of MLG/SiC [39]. The HSD would directly change the C-Si in the interface of MLG/SiC [38], meaning that the variation of C-Si stoichiometric proportion could indirectly reveal the change of the HSD’s level. It can be clearly seen that all fitting components in the XPS spectra change with the hydrogen pressures. The extracted H-C ratios are 8% in 10-2 mbar, 20% in 10-1 mbar and 28% in 102 mbar hydrogenated graphene. And, the Si-C ratio decreases from 36% (10-2 mbar) to 34% (10-1 mbar) and 29% (102 mbar). The increase of the H-C ratio and decrease of Si-C ratio indicated the enhanced level for the HGC and HSD, which would have significant influence on the MLG bandgap as well as the barrier height of MLG/SiC contact. The AFM mapping data of MLG samples’ surfaces with part etched completely have been depicted from figure 2(e)-(g). After the etching, on the SiC surface, we can notice the root mean square (RMS) surface roughness increase with the hydrogen pressure and appearance of aggregated particles, which size also increases with the hydrogen pressure. It can be attributed to the shorter hydrogen atoms free path and higher atoms kinetic energy in higher hydrogen pressure, which will contribute to the fracture of Si-C bonds at high temperature and passivation of HSD. Moreover, at the right side of these maps, the roughness surface could be more clearly observed through MLG at 10-2 mbar than that at 102 mbar, which could be caused by the thicker MLG at higher hydrogen pressure. 7

3.3.Raman characterization In comparison with the as-grown MLG sample (as shown in figure 2 (h)), Raman spectra of these hydrogenated samples depict red shift of D peaks (10-2 mbar~1352cm-1 , 10-1 mbar ~1347 cm-1 and 102 mbar ~1343cm-1), as well as 2D peaks (10-2 mbar~ 2696cm-1 , 10-1 mbar ~2680 cm-1 and 102 mbar ~2672cm-1). It exhibits the hydrogenation influence and the release of MLG’s compressive strain attributed to the HSD component [49].

Figure 2 (a)-(d) and (h) C1s XPS and Raman spectra of 4H-SiC sample’s surface in H2 atmosphere 8

with different pressures. (e)-(g) AFM mapping of these samples which had been grown with MLG on the right side and O2 plasma etched on the left side, as illustrated in maps. (i)-(k) depict Raman mapping of ID/IG and (l)-(n) show FWHM of 2D peaks. All of the scale bars represent 6μm.

In addition, new bands of D+G with peaks at 2927 cm-1 appear after hydrogen treatments. The D+G bands are due to the two-phonon defect-assisted process [43, 50], assigning to the formation of the C-H sp3 as well as the breaking of the translational symmetry of the C=C sp2 bonds in the graphene lattice [43]. On the other hand, the frequency-integrated intensity ratios of D band to G band (ID/IG) can serve as a convenient measurement of the amount of defects in graphitic materials. As the Raman mapping of ID/IG shown in figure 2(i)-(k),

ID/IG of the

above hydrogenated MLG increases with hydrogen pressure, suggesting high defect density induced on MLG’s surface after hydrogenation [43]. As shown in figure 2(l)-(n), 2D bands’ FWHM decrease with hydrogen pressure. Considering the enlargement of the aggregated particles observed in AFM results and the red shift of 2D peaks position in figure 2 (h) with hydrogen pressure, we attribute the decrease of FWHM to the increase of nanographite crystallite size [51]. Along with the increasing of MLG crystallite size, it can be deduced that the growth of defect intensity as well as value of ID/IG may come from the formation of the C-H sp3. 3.4. Schottky barrier height (SBH) The value ( ∅

) of SBH of graphene/4H-SiC heterojunction can be well

described by Schottky-Mott (S-M) limit: , ∅

=Ф

−

, 9

(1)

where Фgr denotes the work function of the graphene (Фgr~ 4.31 eV) on 4H-SiC’s surface [52] and χe denotes the electron affinity of the semiconductor (χe~ 3.7 eV) [53]. The ∅

can be estimated from Фgr which would be modified by the shifting

of Fermi level (EF). In practical, diodes with MLG/4H-SiC junction have been prepared and studied to evaluate the electrical characteristics of junction’s SBH. The forward I-V characteristics of the devices based on three MLG samples are illustrated in figure 3(a) and the schematic diagram of MLG/4H-SiC junction is illustrated as inset.

Figure 3 (a) The forward I-V characteristics of diodes with MLG/4H-SiC junction and the schematic diagram of the fabricated device with Cr/Au (10nm/100nm) pad. (b) The dependence of SBH and ideal factor on hydrogen pressures.

The rectifying behavior of the MLG/4H-SiC junction may be described using thermionic emission theory, which is expressed as ∗∗

= where mm2),

∗∗

exp −

∅

[

denotes current density,

2

(2)

denotes the contact area of the junction (~2

denotes the effective Richardson constant (~146 '() * ) ), T denotes

the operation temperature, + 10)

− 1]

!

denotes the Boltzmann constant(~ 1.38065 ×

/*), 4 denotes the electron charge (~1.602 × 10)67 8)，V denotes the biased 10

voltage and η denotes the ideality factor. A fit of Ln( )-V from a typical MLG/4H-SiC junction displays an adequate linearity under forward bias (0–0.35 V), which allows us to extract the SBH and ideality factor of the Schottky junction. The effective SBH at zero bias can be extracted from interception of the data fitting at low bias voltage. As shown in figure 3(b)，in consideration of the incompele removal of interface features, the estimated zero-bias SBH (≥0.84 eV, EF shifts 0.23 ~0.39 eV below dirac point) come out to be higher than the estimated value (0.61 eV) by equation (1) for the neutral graphene (EF at the dirac point) on 4H-SiC substrate. Moreover, the SBHs of samples in this work is 0.09 ~0.25 eV higher than that of 0.75 eV (EF shifts 0.14 eV below the dirac point) in ref.39. The extra enhancement of SBH would be due to the EF shift (caused by the HGC) below dirac point for 0.23 ~0.39 eV, being similar to the reported works [33, 35]. And, the increase of HGC could further increase work function and then barrier height of MLG/4H-SiC junction [33, 44, 46]. In addition, there were higher SBH value on the end of long sides of hydrogenated SiC sample with more layers of graphene [54]. The hydrogenated MLG sample with higher SBH could show better performance of photodetectors. And, it can be deduced that more layers of hydrogenated graphene would further promote the performance of the photodetectors. Moreover, depending on the intensity ratio of I2D/IG (0.65~10-2 mbar, 0.34~10-1 mbar, 0.04~102 mbar) from figure 2(h), we can estimate EF shift value (0.23 ~0.39 eV) of our samples come out to be lower than that (0.70 eV) of the transferred hydrogenated graphene with the same ratio of I2D/IG on SiO2 [55]. It could be 11

attributed to interface layer of MLG/SiC which pins the Fermi level, leading to the limited EF shift value. The ideality factor (η), which reflects inhomogeneous of MLG/4H-SiC junction and implies the defective contaction, can be extracted from the slope of forward bias of linear region in Ln(J)-V curves, by equation (3): η =

:

(3)

:(<= >)

η can be observed decreasing from 2.4 to 1.4 with the hydrogen pressure. These values are close to the ideal value of unity, which reveal the promising homogeneity of the Schottky barriers with the increase of hydrogen pressure. 3.5 Application and discussion A potential application for hydrogenated MLG on SiC substrate grown by the JHD method was then discussed. As inset in figure 4 (b), an interdigitated finger GSG UV PD was fabricated by standard photolithography and CF4 gas reactive ion etching, using MLG/SiC sample grown in 10-2 mbar hydrogen pressure. The optically active area was 380×480 µm2 , and the fingers were 5 µm wide and 380 µm long with spacing of 5 µm. At MLG/SiC herojunction, high value of stable SBH will exponentially reduce the dark current, and directly improve the performance of GSG UV PD. It could further effectively restrain the dark current by growing the MLG/SiC in higher hydrogen pressure which will result in higher SBH. However, the higher hydrogen pressure will also increase defects of MLG. Therefore, the PDs based on high defect MLG should be operated at relatively low bias voltages to achieve the stable SBHs and control the Fermi level shift |ΔEC | (FLS) in graphene relative to the 12

Dirac point [56].

Figure 4 (a) The diagram of GSG structure. (b) Dark current and photocurrent as a function of bias voltage measured at room temperature for MLG/4H-SiC GSG PD (as the inset in (c)). The photocurrent is measured under 324 nm narrow band UV illumination with 1.1 × 10)D E power. (c) Photocurrent with peak at 324 nm is measured in UV illumination range from 200 to 400 nm under 3V bias voltage.

Figure 4. (a) illustrates the diagram of GSG structure. And, as depicted in figure 4. (b), dark current of this sample cannot be saturated with the increase of bias voltage, which may come from the unstable SBH and thermionic emission diffusion (TED). Here, under reverse bias, the current of GSG UV PD can be derived from equation (2) by TED theory and expressed as [57]

=

F∗ G HI HI JHK

· exp −

where PQ ≈ S= TU , PV =

∗

∅ )|MNO |

[

!

− 1]

(4)

⁄4WX , TU denotes the maximum electric field at

MLG/SiC interface, S= denotes electron mobility and WX denotes effective density of states in conduction band. ∅Z − |ΔEC | decreases with bias voltage for FLS, resulting in unsaturated dark current [56]. And, under the 324 nm illumination with 1.1 × 10)D E power, a photocurrent 10 times more than the dark current is observed. As shown in figure 4 (c), this GSG PD has a spectral response in UV band with peak at 324 nm. At 3V bias voltage, R comes to be 0.9 A/W, corresponding to EQE ~ 345%, 13

where the relationship between EQE and responsivity can be expressed as

R=

\]\×^

(5)

6 _6

As described above, this high value of EQE (>1) can be also ascribed to the effect of FLS and TED. UV photodetector

Responsivities (A/W) at 324nm EQE

Reference

EG/SiC/EG

0.9 @3V

345%

EG/SiC/EG

0.005@5V, 311 nm

2%

Ref.58

EG/SiC PIN

[email protected], 310nm

80%

Ref.59

EG/SiC SBP 2.18@-5V, high dark current 832% EG/SiC SEPT 1.2 @30V, 365 nm, high dark current 408%

Ref.60 Ref.61

Table 1, Comparison of photodetector responsivity in the UV region (concerned on 324 nm) from several reported graphene-SiC UV photodetectors. SBP: Schottky-barrier photodiode; SEPT: Schottky emitter bipolar phototransistor.

By comparing with other Gr-SiC structure photodetector in table 1, R and EQE are much higher than previous works [58, 59] and lower than the reported results [60, 61]. Specially, the illumination power in Ref. 60, 61 (10-3 W and 10-6 W) are much higher than the 1.1×10-8 W in this work and almost have the same magnitudes of light current and dark current. It could be attributed to their vertical structure of electrical field which aggravates effect of FLS and TED and results in much higher unsaturated dark current. Further researches on reducing the effect of FLS and improving the quality of the MLG/SiC interface are needed.

Conclusion A Joule heating decompostion method has been employed to prepare the 14

hydrogenated MLG sample on the surface of semi-insulating 4H-SiC. The Schottky barrier height of MLG/4H-SiC junction could be modulated by different levels of hydrogenation on graphene and the Si dangling bonds of 4H-SiC. The increase of the stoichiometric ratio of C1s for sp3 hybridized carbon, the decrease of C1s for C-Si bonds at the interface and the red shift of 2D peak in the Raman results, along with the increase of hydrogen pressure, revealed the hydrogenation process. The presence of D+G peaks of the Raman results and SBH enhancement of MLG/4H-SiC junction also proved that the hydrogenation level would increase with hydrogen pressure. Moreover, a SBH as high as a 1.0 eV has been achieved, when the hydrogen pressure is up to 102 mbar. Based on the MLG/SiC junction, high performance GSG photodetectors were fabricated, showing high peak responsivity and external quantum efficiency. This work has provided a new approach to prepare epitaxial MLG on 4H-SiC(0001) surface and exploited an enhanced SBH of MLG/4H-SiC junction.

Acknowledgements This work was supported by the Natural Science Foundation of Fujian Province of China [grant number 2018J05113], the Fundamental Research Funds for the Central Universities [grant numbers, 20720190053&20720190049] and the Jiujiang Fund for Distinguished Young Scholars [grant number 2018043]. The authors thank Dingqu Lin and Shaoxiong Wu from the Physics Department of Xiamen University and Chunquan Zhang, Binbin Chen from Pen-Tung Sah Insitute of Micro-Nano Science and Technology of Xiamen University for technical assistance 15

and valuable discussions.

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Research Highlights of “Tuning electronic properties of epitaxial Multilayer-Graphene/4H-SiC( 0001) by Joule Heating Decomposition in hydrogen”

Dear Editor and Reviewers, This paper entitled “Tuning electronic properties of epitaxial Multilayer-Graphene/4H-SiC( 0001) by Joule Heating Decomposition in hydrogen” has three highlights: ► A Joule heating decompostion method has been employed. ► Bandgap of epitaxial multilayers graphene could be opened and modulated by different level of hydrogenation. ► Schottky barrier height as high as a 1.0 eV have been achieved at the 102 mbar hydrogen pressure.

None Conflict of Interest