Hydrogenated amorphous silicon thin films with nanocrystalline silicon inclusions (a/nc-Si:H) have received considerable attention due to reports of electronic properties comparable to hydrogenated amorphous silicon (a-Si:H) coupled with an improved resistance to the light-induced formation of defects. In this study, a/nc-Si:H thin films are synthesized via radio-frequency plasma-enhanced chemical-vapor deposition with helium and hydrogen diluted silane. The plasma conditions were chosen to simultaneously deposit both Si nanocrystallites and an amorphous silicon matrix. This structure has been confirmed by transmission electron microscopy (TEM) studies. Both plasma electronic diagnostics and TEM image analysis of a/nc-Si:H films deposited with and without a temperature gradient between the capacitively coupled reactor electrodes suggest nanoparticle formation in the plasma, as opposed to solid-state nucleation of the nanoparticles in the film. Optical-absorption studies of the a/nc-Si:H films indicate electrical properties comparable to a-Si:H. In particular, the evolution of the films' photoconductivity over light exposure time shows a Staebler-Wronski effect similar to a-Si:H.
Bibliographical noteFunding Information:
This work is supported by NSF under IGERT Grant No. DGE-0114372 and DOE Grant No. DE-FG02-00ER54583. Two of the authors (C.R.P. and C.B.C.) acknowledge support from the 3M Heltzer Endowed Chair and the University of Minnesota Doctoral Dissertation Fellowship. Two other authors (T.J.B. and J.K.) are supported by NREL/AAD-9-18668-13. Four of the authors (S.T., T.J.B., J.K., and U.K.) acknowledge partial support by the NSF MRSEC DMR-0212302. The assistance of John Vinar and Dr. P. C. Taylor at the University of Utah for measurements of the PDS absorption spectra is gratefully acknowledged. Dr. Markus Lentzen and Professor Knut Urban, Research Center Jülich, provided access and assistance with the aberration-corrected HRTEM. Table I. Deposition conditions for the a - Si : H and a ∕ nc - Si : H films. Pressure(mTorr) Flow-rate Si H 4 ∕ He (5:95) (sccm) a Flow-rate H 2 (sccm) Power (W) T rf ∕ T subst (°C) b Material 100 19 0 5 —/250 a - Si : H 1450 40 100 20 250 ∕ 250 a ∕ nc - Si : H 1800 40 100 20 250 ∕ 250 a ∕ nc - Si : H a Denotes Standard cubic centimeter per minute. b Unspecified temperature (—) indicates that the electrode was not heated. Table II. Plasma conditions used in examining the rf signal characteristics. Pressure(mTorr) Flow-rate Si H 4 ∕ He (5:95)(sccm) a Flow-rate H 2 (sccm) Power (W) T rf ∕ T subst (°C) b 100 19 0 5 —/250 250 30 0 5 —/250 600 40 100 20 250 ∕ 250 1100 40 100 20 250 ∕ 250 1500 40 100 20 250 ∕ 250 1800 40 100 20 250 ∕ 250 a In the case of the pristine He and H 2 plasma, the conditions used were identical to the above, with the exception that the Si H 4 ∕ He flow is replaced by pure He. b Unspecified temperature (—) indicates that the electrode was not heated. Table III. Optical-absorption coefficient evaluated at 1.2 eV , Urbach energy, and Tauc optical gap of the a ∕ nc - Si : H and a - Si : H films. Pressure (mTorr) Material α ( 1.2 eV ) cm − 1 E 0 (meV) E g Tauc (eV) 100 a - Si : H 4 a 43 1.69 1450 a ∕ nc - Si : H 3 54 1.77 1800 a ∕ nc - Si : H 2.5 51 1.74 a The α ( 1.2 eV ) value the 100 - mTorr a - Si : H film was estimated from the PDS measurements. FIG. 1. Schematic of the rf-PECVD deposition system. FIG. 2. Typical current and voltage characteristics obtained for the 1500 - mTorr silane plasma conditions (A) and 1500 - mTorr pristine plasma conditions (B). FIG. 3. Effect of Si H 4 to the phase angle between the rf current and voltage, with offset correction, presented as a function of discharge pressure for the pristine and silane plasma conditions. FIG. 4. Effective power dissipated in the plasma discharge P eff relative to the power measured at the output of the rf generator P input plotted for the silane and pristine plasmas as a function of discharge pressure. FIG. 5. High-resolution TEM through-focus image series of a Si nanoparticle. The contrast of the nanocrystal changes systematically as the focus is changed, while the contrast from the amorphous film does not change systematically in these three images. FIG. 6. Dark-field TEM image of a film deposited under the 1450 - mTorr standard a ∕ nc - Si : H conditions (both electrodes heated to 250 ° C ) that contains Si nanocrystals. In this image, the crystals that are diffracting at the position of the objective aperture in the TEM will appear bright; some nanoparticles approximately 5 nm in size are visible. FIG. 7. Dark-field TEM image of a film deposited with the 1450 - mTorr a ∕ nc - Si : H conditions in the presence of a temperature gradient (unheated rf electrode) which appears to be amorphous. No nanocrystals of Si are identifiable in this image. FIG. 8. Photoconductivity measured as a function of light-exposure time for the 100-, 1450-, and 1800 - mTorr films.