Session Tu-C3

a-C and Related Compounds

Chair: Nazir Kherani, University of Toronto

Tu-C3.1 14:00–14:20

Amorphous Carbon Nitride Films Prepared by Hybrid Deposition Technique

Masami Aono (1), Takanori Takeno (2), Toshiyuki Takagi (3), Nobuaki Kitazawa (1) and Yoshihisa Watanabe (1)

1. Department of Materials Science and Engineering, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka Kanagawa, 239-8686, Japan

2. Graduate School of Engineering, Tohoku University, Aoba 6-6-01, Aramaki, Aoba-ku, Sendai 980-8579, Japan

3. Institute of Fluid Science, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan

Amorphous carbon nitride (a-CNx) thin films are a fascinating material with anomalous mechanical and electrical behaviors. Their properties are strongly dependent on deposition techniques [1]. Most a-CNx films prepared by CVD methods have nitrogen concentration of less than 0.3, and then they show a high hardness and a high resistivity. By contrast, a-CNx films prepared by PVD methods have a higher nitrogen concentration. Among the PVD methods, the sputtering technique allows us to make a-CNx films with semiconducting behaviors such as photoconductivity [2]. However, the films show still high resistivity to be used for electronic and optoelectronic devices. While most work has employed RF magnetron sputtering, herein we report upon the fabrication of the a-CNx films by hybrid deposition technique which is combination of RF-generated nitrogen and argon plasma and DC magnetron sputtering of graphite target.

The a-CNx films were deposited on silicon single crystal and quartz glass substrates. We changed nitrogen gas partial pressure (NPP, N2/N2+Ar) for the deposition to prepared a-CNx films with various concentrations. Total gas pressure was kept constant at 1.3 Pa. The films were characterized using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Optical and electrical properties have been also investigated.

Sharp increase of nitrogen concentration (x = N/C) could be observed in low NPP (NPP ~ 0.3). At 0.3 < NPP < 1, the slope of the N/C ratio was decreased. On the other hand, G-peak position in Raman spectra shifted gradually to lower wave number with increasing NPP. The G-peak position in Raman spectra is related to sp3/sp2 ratio [5]. The films in this study were relatively high sp3/sp2 ratio as compared with a-CNx films deposited by RF sputtering. The increasing in N/C ratio and sp3/sp2 ratio leads to increase optical band gaps and electrical resistivity for the films. A similar tendency has been also reported in other literatures, in which a-CNx films are deposited by RF sputtering method [3,4]. On a-CNx films with N/C ratio being equal, however, the resistivity is two orders of magnitude smaller than that of a-CNx film deposited by RF sputtering.

This work provides the high potentiality of reducing the resistivity of a-CNx films through plasma conditions.

[1] S. Muhl and J. M. Mendez, Diamond Relat. Mater., 8 (1999) 1809

[2] M. Aono, T. Goto, N. Tamura, N. Kitazawa, and Y. Watanabe, Diamond Relat. Mater., 20 (2011) 1208

[3] S.E. Rodil, S. Muhl, S. Maca, and A. C. Ferrari, Thin Solid Films, 433 (2003) 119

[4] N. Tamura, M. Aono, H. Kishimura, N. Kitazawa, and Y. Watanabe, Jap. J. Appl. Phys., 51 (2012) 121401

[5] A.C. Ferrari and J. Robertson, Phys. Rev. B16 (2000)14095

Keywords: carbon nitride, hybrid deposition technique, nitrogen partial pressure, resistivity, XPS

Tu-C3.2 14:20–14:40

Why Such High Electrical Resistivity in PECVD Grown Amorphous Hydrogenated Boron Carbide?

Michelle M. Paquette (1), Christopher L. Keck (1), Bradley J. Nordell (1), Thuong D. Nguyen (1), Sudarshan Karki (1), Paul Rulis (1), Nathan A. Oyler (2), Sean W. King (3), and Anthony N. Caruso (1)

1. Department of Physics and Astronomy, University of Missouri-Kansas City, Kansas City, MO 64110

2. Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110

3. Logic Technology Development, Intel Corporation, Hillsboro, OR 97124

Boron carbide is an intriguing solid comprised of boron-based icosahedra connected by three-atom chains; its electron density is delocalized across the icosahedra, which are bound together by very strong intermolecular bonds. The electrical transport mechanism in crystalline or polycrystalline boron carbide, B4C, has remained a controversial topic for decades, its origin rooted in electron deficiency and defects intrinsic to the solid. Experimentally, B4C is a p-type semiconductor with moderate resistivities (10–2–102 Ωcm). PECVD-grown amorphous hydrogenated boron carbide (a-BxC:Hy), however, displays resistivities more than ten orders of magnitude higher, upwards of 1012 Ωcm. This combination of high resistivity with additional properties unique to boron-rich solids such as a very high 10B thermal neutron capture cross section, low density, high hardness, and high chemical stability, render PECVD-grown a-BxC:Hy a suitable candidate for specialized next-generation semiconductor or nanoelectronics applications requiring ultra-low leakage currents, including solid-state neutron detection and low-k dielectric layers in interconnect systems. To advance these applications, understanding and ultimately optimizing the electronic structure and electrical carrier transport properties (e.g. band gap, defect profile, mobility, charge carrier concentration, recombination lifetime, etc.) of a-BxC:Hy is critical. The PECVD-grown amorphous material shares some similarities with its crystalline counterpart, in particular the icosahedral subunit, but it also exhibits differences, including disorder, hydrogenation, and carbon-atom substitution. Herein, measured electrical carrier transport properties of a-BxC:Hy are reported, and models for its physical and electronic structure are used as a basis for interpreting the observed electrical transport behavior.

Keywords: boron-rich solids, charge transport, high-resistivity semiconductor

Tu-C3.3 14:40–15:00

Excimer Laser Crystallization of Amorphous SiCx on Glass

S. Kühnapfel, D. Amkreutz, C. Klimm, M. Reiche, and N. H. Nickel

Helmholtz-Zentrum Berlin für Materialien und Energie, Institut für Silizium-Photovoltaik, Kekulèstr. 5, 12489 Berlin, Germany

Silicon carbide (SiC) is a promising material for the fabrication of optoelectronic devices. For such applications crystalline SiC is favorable, which usually is produced using high temperature processes. Common deposition and growth techniques pose high demands on the usable substrates and direct deposition on preprocessed device structures is typically not possible.

In this paper, we present a route to circumvent this temperature limitation by employing deposition techniques with low thermal budget that offer the advantage to work on low-cost substrates as often used in photovoltaic thin-film technology. However, the poor crystallinity of the as-deposited amorphous silicon-carbide (a-SiCx) films hampers their use in solar cells. An approach to overcome this drawback is the crystallization of the a-SiCx layer using an excimer laser.

In this work, we study the laser induced crystallization of a-SiCx films deposited by physical vapor deposition (PVD) and plasma enhanced chemical vapor deposition (PECVD). The stoichiometry of the layers was varied by adding methane to the PVD process and by changing the silane to methane ratio in the PECVD process. For the crystallization a KrF excimer laser emitting at a wave length of λ = 248 nm was used. The laser fluence, EL, was varied between EL = 100 and 300 mJ/cm2 and the specimens were irradiated with 200 pulses per unit area. The as-deposited and laser annealed layers were characterized using Raman backscattering spectroscopy, energy dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM) and optical absorption spectroscopy. Upon laser crystallization of a-SiCx grown by physical vapor deposition the specimens segregate into carbon- and silicon-rich phases. It is interesting to note that Raman backscattering reveals vibrational modes at 778 and 875 cm–1 that are commonly attributed to 4H-SiC and 6H-SiC, respectively. On the other hand, Raman spectra taken on the a-SiCx deposited by PECVD do not indicated the presence of C–C bonds, although EDX measurements confirm the presence of carbon. After laser crystallization a broad vibrational band at ν = 800 cm–1 appears, which most likely is caused by Si–C and Si–CH3 bonds [1] indicating that the presence of hydrogen significantly influences the amorphous starting material and the resulting poly-SiC films.

[1] H. Wiederer, M. Cardona, and C. R. Guarnieri, phys. stat. sol (b) 92, 99 (1979)

Keywords: silicon carbide, crystallization, low temperature

Tu-C3.4 15:00–15:20

X-ray Photoelectron Spectroscopy Studies on Silicon Carbide Thin Films Prepared by HWCVD

Himanshu S. Jha (1), Asha Yadav (2), M. Singh (1), S. Kumar (3), and Pratima Agarwal (1,2)

1. Department of Physics, Indian Institute of Technology Guwahati, Guwahati, India -781 039

2. Center for Energy, Indian Institute of Technology Guwahati, Guwahati, India -781039

3. Indus Synchrotron Utilization Division, RR Centre for Advanced Technology, Indore, India - 452013

The X-ray photoelectron spectroscopy (XPS) is most reliable technique for the determination of chemical composition and bonding states on thin films surfaces. However, exact quantitative estimation of all the elements present in the film, from XPS analyses is complicated. In this paper, we report the studies of structural and compositional properties of the SiC films deposited by Hot Wire chemical vapor deposition (HWCVD) technique using mixture of hydrogen diluted silane and methane at low substrate temperature (TS) ~ 350°C.

The films are deposited on corning 1737 glass substrate by varying the process pressure (2–5 mbar), keeping other parameters such as TS, silane flow rate, methane flow rate and filament temperature fixed at 350°C, 20 SCCM, 2 SCCM and 1900°C respectively.

The XPS measurements are performed using Mg Kα X-ray source. The measurements are done on samples stored in ambient conditions for 6 months and then after etching the top surface by sputtering of 1 KV Ar+ ions for 15 minutes. All the films have similar wide and narrow scan XPS spectra. The peaks present in the spectra are the signature of different states of elements of Si (2p), Si (2s), C (1s), O (1s) and O (KLL) at ~100, 150, 285, 532 and 740 eV respectively. The presence of O (1s) peak (532 eV) in all the films is possibly due to residual oxygen in the growth chamber. These analyses confirm that the Si-C bonds are the main bonds in the films and chemical composition of SiC films are nearly stoichiometric with almost equal share of silicon and carbon.

Acknowledgments: The work reported here is supported by Board for Research in Fusion Science & Technology (BRFST), Institute of Plasma Research, Gandhinagar, India. We would also like to acknowledge Dr. D. M. Phase, UGC-DAE Consortium for Scientific Research, Indore, for XPS measurements.

Keywords: XPS, silicon carbide, HWCVD