Anomalously high thermal conductivity of amorphous Si deposited by hot-wire chemical vapor deposition

Ho-Soon Yang, David G. Cahill, X. Liu, J. L. Feldman, R. S. Crandall, B. A. Sperling, and J. R. Abelson

Phys. Rev. B 81, 104203 – Published 15 March 2010

Abstract

The thermal conductivities of thin films of amorphous Si (a-Si) deposited by hot-wire chemical vapor deposition (HWCVD) are measured by time-domain thermoreflectance (TDTR). Amorphous Si samples prepared at the National Renewable Energy Laboratory (NREL) show an anomalous enhancement in thermal conductivity compared to other forms of a-Si and compared to the prediction of the model of the minimum thermal conductivity. The thermal conductivity of the NREL HWCVD a-Si samples also decreases with increasing frequency of the temperature fields used in the experiment. This frequency dependence of the thermal conductivity is nearly identical to the results of our previous studies of crystalline semiconductor alloys; a comparison of the frequency dependence to a phonon-scattering model suggests that Rayleigh-type scattering controls the mean-free path of $\sim 5 meV$ phonons in this material. Amorphous Si films prepared at University of Illinois (U. Illinois) do not show an enhanced thermal conductivity even though Raman vibrational spectra of the U. Illinois and NREL samples are nearly identical. Thus, the thermal conductivity of a-Si depends on details of the microstructure that are not revealed by vibrational spectroscopy and measurements by TDTR provide a convenient method of identifying novel microstructures in amorphous materials.

Received 20 November 2009

DOI:https://doi.org/10.1103/PhysRevB.81.104203

Authors & Affiliations

Ho-Soon Yang

Department of Physics, Pusan National University, Pusan 609-735, Korea

David G. Cahill^*

Department of Materials Science and Engineering, and Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA

X. Liu

Naval Research Laboratory, Washington, DC 20375, USA

J. L. Feldman

Center for Computational Materials Science, Naval Research Laboratory, Washington, DC 20375, USA and Department of Computation and Data Sciences, George Mason University, Fairfax, Virginia 22030, USA

R. S. Crandall

National Renewable Energy Laboratory, Golden, Colorado 80401, USA

B. A. Sperling and J. R. Abelson

Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801, USA

^*d-cahill@uiuc.edu

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Vol. 81, Iss. 10 — 1 March 2010

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Images

Figure 1
Temperature dependence of the thermal conductivity of two NREL a-Si films (circles: H14 and triangles: H35) measured with a modulation frequency of $f = 1.1 MHz$ (open symbols) and $f = 9.8 MHz$ (filled symbols). Data for cubic YSZ and ${a-SiO}_{2}$ (right-pointing triangles) are included for comparison. Solid lines are the thermal conductivity of YSZ and ${a-SiO}_{2}$ measured previously with the $3 ω$ method, see Ref. 1. Representative error bars of $\pm 10 %$ for $f = 9.8 MHz$ and $\pm 15 %$ for $f = 1.1 MHz$ are shown in the data for sample H14.Reuse & Permissions
Figure 2
Thermal conductivity of an NREL a-Si film (H14), YSZ, and ${SiO}_{2}$ as a function of the modulation frequency of the pump beam used in the TDTR measurement. For sample H14 and for ${SiO}_{2}$ , the open symbols are for data taken at $T = 115 K$ and filled symbols are for room temperature. For YSZ, open symbols are for data taken at 154 K and filled symbols are for 328 K. The dashed line shows a theoretical estimate of the frequency dependence based on Eq. (1).Reuse & Permissions
Figure 3
Variations in the properties of HWCVD a-Si films prepared at U. Illinois (filled right-pointing triangles) as a function of the substrate temperature during film deposition. (a) Thermal conductivity measured at room temperature and (b) longitudinal speed of sound measured at room temperature. Thermal conductivities of NREL samples H1060 (diamond), H969 (open square), H819 (open circle), H14 (filled circle), and H34 (filled up pointing triangle) are included in (a) for comparison. The modulation frequency used to acquire the thermal conductivity in (a) is 9.8 MHz. The uncertainty of the thermal conductivity measurements is $\approx \pm 10 %$ . In (b), the dashed line is the averaged longitudinal speed of sound for polycrystalline Si.Reuse & Permissions
Figure 4
(Color online) (a) Raman spectra of U. Illinois a-Si films deposited at different substrate temperatures; from top to bottom, the curves are for growth temperatures of 114, 200, 300, 400, 500, and $600 ° C$ . The spectra were collected at room temperature with 488 nm excitation. The overall peak height increases gradually; a broad peak at $615 {cm}^{- 1}$ appears as the growth temperature decreases. (b) Raman spectra of NREL a-Si film H14 compared to the U. Illinois a-Si film grown at $400 ° C$ . Data for sample H35 (not shown) are indistinguishable from data for sample H14. (c) Raman spectra of NREL a-Si films H1060 (red), H819 (blue), and H969 (black). Raman spectra of H1060 and H819 are similar to the spectra for H14 and H35 except for a weak peak near $615 {cm}^{- 1}$ . Sample H969 (80% nanocrystalline a-Si) shows a narrow peak at $513 {cm}^{- 1}$ .Reuse & Permissions

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