graphite

    Growth and Thermoelectric Properties of Nitrogen-doped Diamond/Graphite Armin Haase, Alexandra Peters, Stefan Rosiwal PII: DOI: Reference:

S0925-9635(15)30069-8 doi: 10.1016/j.diamond.2015.10.023 DIAMAT 6494

To appear in:

Diamond & Related Materials

Received date: Revised date: Accepted date:

2 July 2015 16 October 2015 24 October 2015

Please cite this article as: Armin Haase, Alexandra Peters, Stefan Rosiwal, Growth and Thermoelectric Properties of Nitrogen-doped Diamond/Graphite, Diamond & Related Materials (2015), doi: 10.1016/j.diamond.2015.10.023

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

ACCEPTED MANUSCRIPT Growth and Thermoelectric Properties of Nitrogen-doped Diamond/Graphite WTM, Universit¨ at Erlangen-N¨ urnberg, Martensstr. 5, 91058 Erlangen, Germany b Energie Campus N¨ urnberg, F¨ urther Str. 250, 90429 N¨ urnberg, Germany

SC

RI

a

PT

Armin Haasea,b,∗, Alexandra Petersa , Stefan Rosiwala

Abstract

TE

D

MA

NU

To investigate the growth and the thermoelectric properties of nitrogen-doped diamond, samples were grown by microwave plasma chemical vapor deposition with varying nitrogen flows as well as with a high nitrogen flow and varying methane flows. The samples were characterized according to their morphology, phase composition, electrical conductivity, and Seebeck coefficient. It was found that an increased nitrogen flow leads to a higher fraction of sp2 -bond carbon. Furthermore, a structure consisting of graphene nanowalls which exhibits many cavities is created. For nitrogen flows above 70 sccm, the electrical conductivity increases abruptly and significantly from less than 0.3 mS to more than 4.3 mS . A lower methane flow inverts this development. It was not possible to find a clear dependency of the Seebeck coefficient on the nitrogen flow. Seebeck coefficients were measured between −17 µV at 23 ◦C K and −34 µV at 110 ◦C. K

AC CE P

Keywords: CVD, diamond, graphite, doping, nitrogen, thermoelectricity, Seebeck 1. Introduction

Thermoelectric materials already in use are often toxic and expensive as they contain rare elements. Their temperature range is also limited, making them unsuitable for high-temperature applications. A recently discussed solution to these problems is diamond. Its creation by means of chemical vapor deposition (CVD) meanwhile is a standard procedure which would enable a large-scale application in the future. Early measurements have shown the great prospects of this material in the field of thermoelectricity [1]. Due to its covalent bonds, single-crystal diamond is the best of all known electrical isolators, making *WTM, Universit¨ at Martensstr. 5, 91058 [email protected], erlangen.de

Erlangen-N¨ urnberg, Erlangen, Germany, http://www.wtm.uni-

Preprint submitted to Diamond and Related Materials

it essential to find dopants. Especially with nconduction this task has turned out to be very challenging. Nitrogen is logical as a dopant due to its additional valence electron in comparison with carbon. It is known as single-substitutional and it matches the crystal lattice of diamond well because of its similar atomic radius. Distortion by bigger dopants is generally a problem with diamond due to its tight lattice and it can lead to graphitization. Another important advantage of nitrogen is its easy handling during the doping and deposition processes. As it is gaseous at room temperature and therefore also at typical fabrication temperatures, it can simply be added to the gas phase. The electrical properties of CVD diamond have been investigated on several occasions (cf. [2]). The addition of nitrogen and its influence on conductivity and the type of conNovember 7, 2015

ACCEPTED MANUSCRIPT show grain sizes up to the order of magnitude of millimeters. Measuring the Seebeck coefficient, which is usually only done to determine the type of conductivity, also reveals the thermoelectric capacity of N-doped diamond.

duction have also been the subject of research. Usually the samples are made by microwave plasma CVD (MWCVD) [3–7], less often by hot-filament CVD (HFCVD) [8]. Rather an exception is the ion implantation into MWCVD or natural diamond [9]. Argon is typically used for MWCVD; either methane (CH4 ) or acetylene (C2 H2 ) is typically used as a carbon source. Using hydrogen instead of argon, as for this paper, is seldom done [10]. Comparing the results of different authors is complicated not only due to their different preparation methods but also the variety of names given to the grown carbon materials. Using Ar and CH4 they vary from carbon nanowalls [3] to ultra-nano crystalline diamond (UNCD) [4–6, 11] and diamond-like carbon (DLC) [7]. Most authors describe their samples as Ndoped [5, 6, 9–11]. Still, the question arises if N2 in the gas phase is equivalent to doping diamond with it. Even if that is the case, the impact on the conductivity needs to be explained. According to [9] N2 in the gas phase leads to conductivity. [3], [4] and [6] come to the same result and explain it with grain boundaries. [11] and [5] determine n-type conduction using Hall and Seebeck measurements. The latter trace it back to sp2 and not to doping with nitrogen. However, [10] claim that single-substitutional N+ ions compensate for the conductivity due to boron. In contrast, [8] find p-type conduction which becomes n-type through oxidization. What is generally missing is the ratio between sp2 and sp3 , though this information can be easily obtained by Raman measurements. The unusual spectra of [7] are not used to determine that ratio. Consequently, it is necessary to understand the influence that nitrogen in the gas phase has on the growth of diamond for further comprehension. This paper therefore deals with diamond deposited in MWCVD. The hydrogen used for this process guarantees a much higher quality than is obtainable with argon. The likelihood of creating sp2 is reduced due to the higher etching rate in comparison to sp3 while the micro-crystalline structure can

PT

2. Experimental methods

AC CE P

TE

D

MA

NU

SC

RI

2.1. Film growth To grow the carbon films, graphite disks were used as substrates. They had a diameter of 56 mm and were polished with sandpaper with a grit of 320. Before deposition they were dip-seeded in a dispersion of nano diamonds in ethanol. In order to do so the commercially available dispersion Andante by Carbodeon was diluted with ethanol at a ratio of 1:20. The substrates were dried afterwards in flowing air at room temperature. As free-standing diamond films are the subject of the investigation, the substrate was removed after the deposition. For that purpose the backsides of the samples were blasted with SiC powder with a grain size of 320 µm or less. The remaining films were cleaned in ethanol in an ultrasonic bath and again dried in flowing air. The deposition itself took place in an MWCVD reactor AX 6350 by ASTeX. The chamber was evacuated to a pressure of less than 1.33 mbar. As pressure and power in the reactor are not independent of each other, during the growth process the former was kept at 133 mbar and the latter at 3500 W. A constant hydrogen flow of 10 sccm was combined with varying nitrogen and methane flows. First, a series of depositions with increasing nitrogen content and a constant methane flow of 5 sccm was performed. The N2 flow was increased in steps of 10 sccm beginning at 0 sccm and ending at 100 sccm. In a second series, with a constant N2 flow of 100 sccm the CH4 flow was reduced from 5 sccm to 1 sccm in steps of 1 sccm in order to decrease the carbon content of the process. The other parameters were left unchanged (H2 10 sccm, 3500 W, and 133 mbar).

2

ACCEPTED MANUSCRIPT The density was determined with a helium pycnometer AccuPyc II 1340 by micromeritics.

2.2. Characterization methods The morphology of the films deposited as described above were characterized by scanning electron microscopy (SEM) using a Quanta 450 by FEI and an acceleration voltage of 20 kV. If necessary, non-conductive or poorly conductive samples were covered with a thin gold film. To determine the ratio of sp3 -bonded carbon to non-sp3 -bonded, Raman measurements were performed. The laser used for that has a wavelength of λ = 514.5 nm. The spectrometer itself is a Ramascope 2000 by Renishaw. In a stressless sample, the diamond peak can 1 , the disordered graphite be found at 1332 cm 1 peak at 1350 cm , the crystalline graphite peak 1 1 from 1550 cm to 1580 cm and the trans-poly1 1 to 1150 cm and acetylene peaks from 1140 cm 1 1 from 1450 cm to 1480 cm . As the ratio of the cross sections of sp3 -bonded and sp2 -bonded carbon for 514.5 nm was found to be between 46 and 88 [12], it is necessary to assess the Raman signals. With an appropriate spectrum it is possible to calculate the diamond fraction [13]. The resistivity was measured with four-terminal sensing using osmium tips and a PM 6303. The average layer thickness was determined with a Zeiss Axiophot. However, the surface can be rough in comparison to the thickness as a result of uneven growth, making the thickness more often an estimation than an exactly measured value. The distance between the measuring tips, however, is in any case large compared to the film thickness d, thus making it necessary to use a correction to calculate the conductivity [14]:

3. Results and discussion

AC CE P

TE

D

MA

NU

SC

RI

PT

3.1. Morphology and phases Different gas flows cause different sample morphologies which are even visible to the naked eye. In the microscopic range, SEM images reveal to what extent this happens. Figure 1 shows the surface of two samples which were grown with ΦN2 = 0 sccm (left side) and ΦN2 = 80 sccm (right side). The other parameters were kept constant (ΦCH4 = 5 sccm, ΦH2 = 10 sccm, P = 3500 W, p = 133 mbar). At a lower magnification, both samples look cauliflower-like. The higher magnification shows a granular and dense surface for the nitrogen-free growth. With an N2 flow of 80 sccm, this changes dramatically. The sample is not bulky anymore but has numerous cavities between carbon structures which appear needle-like from above.

σ=

I ln 2 1 · · U π d

(1) Figure 1: SEM image showing the influence of nitrogen flow on surface morphology. The left side was grown with ΦN2 = 0 sccm, the right side with ΦN2 = 80 sccm (constant: ΦCH4 = 5 sccm, ΦH2 = 10 sccm, P = 3500 W, p = 133 mbar).

For the Seebeck measurements, equipment developed by us was used that allows the cold end to be kept at approximately 2 ◦C while the warm end can be heated up to 300 ◦C. The voltage between the hot and cold ends A similar change can be observed with an inas well as the temperatures were measured at creasing methane flow and a relatively high nitrogen flow of ΦN2 = 100 sccm. ΦH2 , P , and the sample surface. p were kept constant again. Figure 2 shows 3

ACCEPTED MANUSCRIPT

SC

RI

PT

the sample surfaces for ΦCH4 = 1 sccm (left side) and ΦCH4 = 4 sccm (right side). With the low methane flow it is denser and has a coarsely granular structure. As the only difference to the sample on the right side of figure 1 is a CH4 flow of 4 sccm instead of 5 sccm, again a needle-like structure appears.

MA

NU

Figure 3: SEM image showing the broken edge of a sample with graphene nanowalls which appear needlelike from above (ΦN2 = 100 sccm, ΦCH4 = 3 sccm, ΦH2 = 10 sccm, P = 3500 W, p = 100 mbar).

D

D peak has the higher intensity. The more nitrogen there is in the gas phase, the more the growth of disordered graphite is preferred. The fact that the diamond peak is hardly visible makes it difficult to obtain information from the Raman spectra about the ratio of sp2 and sp3 -bonded carbon. Still, its very top can be seen for the two curves which indicate lower nitrogen flows, which makes it reasonable to conclude that an increasing nitrogen flow supports the growth of graphite. Calculating the ratio of the sp2 and sp3 content as done by others [13] does not make sense at this point. The same is true for Raman spectra for different methane flows (figure 5). While the tPA peak does not seem to be affected by the increase of the CH4 flow from 1 sccm to 4 sccm in steps of 1 sccm, the diamond and graphite growth is. With low carbon content in the gas phase (1 and 2 sccm), the peak at 1 1332 cm is still visible though most of its area 1 is identical with the sp2 peak at 1350 cm . For methane flows of 3 and 4 sccm it cannot be identified anymore. Taking into consideration the different cross sections both carbon phases have for an excitation wavelength of 514.5 nm, there should nevertheless be a certain amount of diamond in the sample. How-

AC CE P

TE

Figure 2: SEM image showing the influence of the methane flow on the surface morphology. The left side was grown with ΦCH4 = 1 sccm, the right side with ΦCH4 = 4 sccm (constant: ΦN2 = 100 sccm, ΦH2 = 10 sccm, P = 3500 W, p = 133 mbar).

This structure is indeed described as “nanorods” [15, 16]. The SEM image of the broken edge of a sample which is shown in figure 3 reveals that this is not the case. Obviously, what appears needle-like is only the upper edge of graphene nanowalls growing away from the substrate. An interesting aspect which will be discussed later are the cavities between those graphene nanowalls. They have an influence on the density of the sample. Figure 4 shows the Raman spectra of samples grown with nitrogen flows of 20 sccm, 40 sccm, 80 sccm, and 100 sccm. The intensity in arbitrary units is plotted above the wavenumber 1 in cm . It is obvious that the tPA peak gets smaller with increasing N2 flow. The ratio between the intensity of the D and the G peak of graphite also changes. While for 20 and 40 sccm the G peak is bigger, for 80 sccm they have the same height. Above that level the 4

ACCEPTED MANUSCRIPT

SC

RI

PT

with ΦCH4 = 5 sccm, ΦH2 = 10 sccm, P = 3500 W, and p = 133 mbar. For ΦN2 = 0 sccm, the density is 3.32 cmg 3 and therefore close to that of single-crystal diamond of 3.52 cmg 3 . Though the samples measured for this paper do not show a non-ambiguous dependency, it is obvious that the density decreases as nitrogen flow increases. In particular, it drops down for flows of 80 sccm and above. This is the range where the graphene nanowalls structure finally evolves and the lower density is caused not only by more graphite but also by cavities within the sample.

NU

Figure 4: Raman spectra of samples grown with varying nitrogen flows (constant: ΦCH4 = 5 sccm, ΦH2 = 10 sccm, P = 3500 W, p = 133 mbar).

AC CE P

TE

D

MA

ever, to calculate it is not possible. The second effect which an increase of the CH4 flow has is the same shift of the intensity of the D and the G peak of graphite that was already observed for an increasing N2 flow. Starting with a higher G peak, the more methane there is in the gas phase, the higher the D peak becomes until it surpasses the G peak for 4 sccm.

Figure 6: The influence of nitrogen flow on the density of the sample (constant: ΦCH4 = 5 sccm, ΦH2 = 10 sccm, P = 3500 W, p = 133 mbar). An increasing N2 flow leads to decreasing density.

The conclusion that is to be drawn from SEM images and the Raman spectra is the following: Higher nitrogen flow as well as higher methane flow (with simultaneously high nitrogen flow of 100 sccm) result in a preference of graphite growth and lead to a graphene nanowalls structure with cavities. When investigating the electrical conductivity one Figure 5: Raman spectra of samples grown with therefore must take these changes into convarying methane flows (constant: ΦN2 = 100 sccm, sideration.

ΦH2 = 10 sccm, P = 3500 W, p = 133 mbar).

3.2. Conductivity and thermoelectric properties The changes caused by higher nitrogen flow in the growth process are reflected in the conductivity measurements as figure 7 shows. The conductivity in mS is plotted over the nitrogen

As shown, nitrogen does not only affect the carbon phase whose growth is preferred but also the morphology. Both result in a changing apparent density of the samples as can be seen in figure 6. All layers were deposited 5

ACCEPTED MANUSCRIPT firms the idea that sp2 is responsible for the conductivity of the samples. Within the measured temperature range the Seebeck coefficient is negative. Thus it differs clearly from the thermoelectric properties of boron-doped crystalline diamond [19]. The absolute value increases along with the temperature as expected for all samples and lies between −17 . The average slope of all curves and −25 µV K is very similar and linear. Only the samples grown with ΦN2 = 10 sccm and ΦN2 = 20 sccm start at 25 µV at 25 ◦C and go up to 34 µV at K K ◦ 107 C. A physical explanation for this difference is difficult to find. Also it is hard to identify any influence the nitrogen flow has on the Seebeck coefficient in general. This is especially remarkable as the electrical conductivity σ has the abrupt and significant increase for samples grown with ΦN2 ≥ 70 sccm. A higher σ usually results in increased heat conductivity for semiconductors and a lower Seebeck coefficient would be expected. Further experiments need to be performed to clarify the reasons for this unusual behavior.

AC CE P

TE

D

MA

NU

SC

RI

PT

flow in sccm. For N2 flows of 50 sccm and below, the conductivity is lower than 0.3 mS . Above this threshold value an abrupt and significant increase appears. For 70 and 80 sccm it is above 4.3 mS , for 90 sccm even above 6.9 mS . These are exactly the same values for which the density drops and for which the diamond peak in the Raman spectrum is completely covered by the peak of disordered graphite. We therefore conclude that the lower electrical resistivity is the result of higher graphite content in the samples. Other investigations show that the graphite can primarily be found at the grain boundaries [17]. It is possible that above a certain nitrogen flow (with the other parameters used in this paper above 70 sccm) the growth is affected in such a way that there is continuous sp2 phase throughout the sample. Thus electrical conductivity is rather the outcome of graphitization than nitrogen doping. A deep donor level of 1.7 eV, as found by [2], would explain these results very well.

Figure 8: The influence of nitrogen flow on the Seebeck coefficient of the sample (constant: ΦCH4 = 5 sccm, ΦH2 = 10 sccm, P = 3500 W, p = 133 mbar).

Figure 7: The influence of the nitrogen flow on the conductivity of the sample (constant: ΦCH4 = 5 sccm, ΦH2 = 10 sccm, P = 3500 W, p = 133 mbar). A N2 flow of more than 50 sccm leads to an abrupt and significant increase.

4. Conclusion No obvious correlation can be found for the Seebeck coefficient, as can be seen in Figure 8, where the Seebeck coefficient is plotted for samples with varying nitrogen flow above the temperature. It has the same order of magnitude as the one of graphite [18] which con-

The growth and the thermoelectric properties of MWCVD-grown diamond with nitrogen in the gas phase was investigated. The higher the nitrogen flow in the gas phase was, the the higher the fraction of graphite in the 6

ACCEPTED MANUSCRIPT sample becomes and the more the structure consists of graphene nanowalls separated by cavities. As high nitrogen flow also leads to an increase in conductivity, we conclude that the graphite, and not the nitrogen doping of the diamond, is responsible for it. The Seebeck coefficient is similar for all samples including those with high electrical conductivity. The reason for this is not clear yet. With these results, n-conducting diamond for thermoelectric applications based on nitrogen in the gas phase seems to be a rather unsuitable material. The high graphite fraction and the comparatively low Seebeck coefficient make it necessary to look for other elements which can be used as dopants.

[5] T. Ikeda, K. Teii, C. Casiraghi, J. Robertson, A. C. Ferrari, Effect of the sp[sup 2] carbon phase on n-type conduction in nanodiamond films, Journal of Applied Physics 104 (7) (2008) 73720.

Acknowledgements

[8] R. Horiuchi, K. Okano, N. Rupesinghe, M. Chhowalla, G. Amaratunga, Seebeck measurements of n-doped diamond thin film, physica status solidi (a) 193 (3) (2002) 457–461.

SC

RI

PT

[6] P. Achatz, O. A. Williams, P. Bruno, D. M. Gruen, J. A. Garrido, M. Stutzmann, Effect of nitrogen on the electronic properties of ultrananocrystalline diamond thin films grown on quartz and diamond substrates, Physical Review B 74 (15).

MA

NU

[7] Z. Seker, H. Ozdamar, M. Esen, R. Esen, H. Kavak, The effect of nitrogen incorporation in dlc films deposited by ecr microwave plasma cvd, Applied Surface Science 314 (2014) 46–51.

D

The authors gratefully acknowledge use of the services and facilities of the Energie Campus N¨ urnberg and financial support through the ”Aufbruch Bayern” initiative of the state of Bavaria.

TE

AC CE P

References

[9] M. Hasegawa, Y. Yamamoto, H. Watanabe, H. Okushi, M. Watanabe, T. Sekiguchi, Characterisation of nitrogen-implanted cvd homoepitaxial diamond, Diamond and Related Materials 13 (4-8) (2004) 600–603.

[1] H. J. Goldsmid, C. C. Jenns, D. A. Wright, The thermoelectric power of a semiconduct- [10] V. I. Polyakov, A. I. Rukovishnikov, N. M. ing diamond, Proceedings of the Physical Rossukanyi, V. G. Ralchenko, Electrical Society 73 (3) (1959) 393–398. properties of thick boron and nitrogen contained cvd diamond films, Diamond and Re[2] S. Koizumi, C. Nebel, M. Nesladek, Physics lated Materials 10 (3-7) (2001) 593–600. and Applications of CVD Diamond, WileyVCH, Hoboken, 2008. [11] O. A. Williams, S. Curat, J. E. Gerbi, D. M.

Gruen, R. B. Jackman, n-type conductivity [3] K. Teii, S. Shimada, M. Nakashima, in ultrananocrystalline diamond films, ApA. T. H. Chuang, Synthesis and electrical plied Physics Letters 85 (10) (2004) 1680. characterization of n-type carbon nanowalls, Journal of Applied Physics 106 (8) (2009) [12] N. Wada, S. A. Solin, Raman efficiency mea84303. surements of graphite, Physica B+C 105 (13) (1981) 353–356. [4] S. Bhattacharyya, O. Auciello, J. Birrell, J. A. Carlisle, L. A. Curtiss, A. N. Goyette, [13] F. Silva, A. Gicquel, A. Tardieu, P. Cledat, D. M. Gruen, A. R. Krauss, J. Schlueter, T. Chauveau, Control of an mpacvd reactor A. Sumant, P. Zapol, Synthesis and charfor polycrystalline textured diamond films acterization of highly-conducting nitrogensynthesis: role of microwave power density, doped ultrananocrystalline diamond films, Diamond and Related Materials (5) (1996) Applied Physics Letters 79 (10) (2001) 1441. 338–344.

7

ACCEPTED MANUSCRIPT [14] D. K. Schroder, Semiconductor material and device characterization, 3rd Edition, Wiley, Piscataway, NJ, Hoboken, N.J, 2006.

RI

PT

[15] A. R. Sobia, S. Adnan, A. Mukhtiar, A. A. Khurram, A. A. Turab, A. Awais, A. Naveed, Q. J. Faisal, H. Javaid, G. J. Yu, Effect of nitrogen addition on hydrogen incorporation in diamond nanorod thin films, Current Applied Physics 12 (3) (2012) 712– 717.

NU

SC

[16] S. A. Rakha, G. Yu, J. Cao, S. He, X. Zhou, Diamond-graphite nanorods produced by microwave plasma chemical vapor deposition, Diamond and Related Materials 19 (4) (2010) 284–287.

D

MA

[17] S. Sobolewski, M. A. Lodes, S. M. Rosiwal, R. F. Singer, Surface energy of growth and seeding side of free standing nanocrystalline diamond foils, Surface and Coatings Technology 232 (2013) 640–644.

AC CE P

TE

[18] P. L. Walker, L. G. Austin, J. J. Tietjen, Oxgen chemisorption effects on graphite thermoelectric power, Chemistry and Physics of Carbon (1) (1966) 327–365. [19] M. Engenhorst, J. Fecher, C. Notthoff, G. Schierning, R. Schmechel, S. M. Rosiwal, Thermoelectric transport properties of boron-doped nanocrystalline diamond foils, Carbon 81 (2015) 650–662.

8