Thermally Induced Nano-Structural and Optical Changes of nc-Si:H Deposited by Hot-Wire CVD
We report on the thermally induced changes of the nano-structural and optical properties of hydrogenated nanocrystalline silicon in the temperature range 200–700 °C. The as-deposited sample has a high crystalline volume fraction of 53% with an average crystallite size of ~3.9 nm, where 66% of the total hydrogen is bonded as ≡Si–H monohydrides on the nano-crystallite surface. A growth in the native crystallite size and crystalline volume fraction occurs at annealing temperatures ≥400 °C, where hydrogen is initially removed from the crystallite grain boundaries followed by its removal from the amorphous network. The nucleation of smaller nano-crystallites at higher temperatures accounts for the enhanced porous structure and the increase in the optical band gap and average gap.
KeywordsHot-wire CVD Quantum size effects Nano-crystallite Optical band gap
Hydrogenated nanocrystalline silicon (nc-Si:H) has been the subject of intense scientific and technological interest over the past decade, mainly due to its reduced photo-induced degradation , efficient visible photoluminescence , tailored optical band gap , increased conductivity and greater doping efficiency . It has been highlighted that these unique features are a direct cause of the quantum size effects of the silicon nano-crystallites. These improvements make nc-Si:H a potential candidate for application in photovoltaic and opto-electronic devices [5, 6].
The hot-wire chemical vapour deposition (HWCVD) technique, based on the catalytic decomposition of the precursor gasses by a heated transition metal filament, has been established as a viable deposition technique for nc-Si:H thin films [6, 7]. The structural and opto-electronic properties of the thin films are dependent on the deposition parameters, of which the hydrogen dilution and substrate temperature are the most crucial. It has been established that the etching effect of atomic hydrogen, created by the catalytic decomposition of H2, is responsible for the termination of weak Si–Si bonds from the surface and sub-surface regions and that the nucleation of the nano-crystallites are improved by increasing the hydrogen dilution [7, 8, 9, 10]. It has also been reported that the hydrogen dilution during deposition determines the concentration and the distribution of hydrogen in nc-Si:H, which is closely related to the nano-structural features; i.e. crystallite size and crystalline volume fraction [11, 12, 13, 14]. These nano-structural features eventually determine the optical properties of the material. In particular, the quantum size effects of the Si nano-crystallites and the hydrogen concentration have a strong correlation with the optical band gap [15, 16].
An investigation into the role of hydrogen in nc-Si:H is therefore crucial for the understanding of its relation to the nano-structure and the optical properties. In this contribution, we investigate the effects of the hydrogen concentration and bonding configuration in nc-Si:H deposited by HWCVD on the nano-structural features and the optical properties. The hydrogen concentration and bonding configuration were controlled by post-deposition isochronal annealing.
The nc-Si:H thin film was deposited by the HWCVD process simultaneously on single-side polished 〈100〉 crystalline silicon and Corning 7059 glass substrates, using a mixture of 4 sccm SiH4 and 26 sccm H2 decomposed by seven parallel tungsten filaments, 15 cm apart and 36 cm away from the substrates. A detailed description of the experimental set-up is given elsewhere [17, 18]. The filament temperature, substrate temperature and deposition pressure were fixed at 1600 °C, 420 °C and 60 μbar, respectively. The as-deposited nc-Si:H thin film was ~1140 nm-thick, as measured using a Veeco® profilometer.
Subsequent annealing was performed under high-purity, flowing N2gas in a tube furnace at annealing temperatures (TA) ranging from 200 to 700 °C in 100 °C increments. The N2flow rate, heating rate and dwell time for all temperatures amounted to 300 sccm, 10 °C/min and 30 min, respectively. After each annealing temperature, the thin film was allowed to cool to room temperature in the tube furnace, while maintaining the N2flow rate. Thereafter the required analytical techniques were performed.
Fourier transform infrared (FTIR) absorption spectra were collected in transmission geometry from 400 to 4000 cm−1with a spectral resolution of 1 cm−1, using a Perkin-Elmer Spectrum 100 FTIR spectrophotometer. The structural properties were investigated using a Jobin-Yvon HR800 micro-Raman spectrometer in backscattering geometry at room temperature. The Raman spectra were collected in the region 100–1000 cm−1with a spectral resolution of 0.4 cm−1, using an excitation wavelength of 514.5 nm. X-ray diffraction (XRD) spectra were collected in reflection geometry at 2θ-values ranging from 10 to 90° with a step size of 0.02°, using a Phillips PW 1830 X-ray powder diffractometer operating at 45 kV and 40 mA. Copper Kα1radiation with a wavelength of 1.5406 Å was used as the X-ray source. Optical transmission spectra were measured from 200 to 900 nm with a spectral resolution of 1 nm, using a Perkin-Elmer LAMDA 750S UV/VIS spectrophotometer.
Results and Discussion
To quantify the fraction of H bonded on the surface of nano-crystallites in nc-Si:H, we define a structure factor, Open image in new window where I denotes that integrated intensity of each decomposed peak. The total bonded hydrogen concentration (CH) was estimated from the integrated absorption of the 640 cm−1 rocking mode using previous reported procedures [26, 27]. In the as-deposited state, CH amounts to ~2 at.%, characteristic for nc-Si:H deposited with high hydrogen dilution [16, 28], where ~66% thereof is bonded on the surface of the nano-crystallites. We propose that this relatively high value for Rs is indicative of a high crystalline volume fraction.
Crystallite size, crystalline volume fraction and optical properties after specific annealing temperatures
The thermally induced nano-structural changes of the nc-Si:H thin film can be interpreted as follows, based on the variation of the Si–Hx bonding and the crystalline character. In the as-deposited state the crystalline volume fraction is relatively large and therefore the majority of H is bonded to the surface of the nano-crystallites. The nano-structural properties are stable at temperatures below 400 °C, attributed to its large crystalline volume fraction. An initial increase in the native crystallite size is observed after annealing at 400 °C, resulting in the removal of hydrogen from the grain boundaries. It is also feasible that smaller crystallites have coalesced into larger crystallites. At higher temperatures, hydrogen is removed preferentially from the amorphous phase, indicative of the nucleation of smaller nano-crystallites of size <3 nm in the amorphous network , undetected by Raman spectroscopy and XRD.
where EM and ED is the average gap and dispersion energy, respectively. The plot of 1/n2(ν)-1] versus (hν)2 allows for the determination of EMED and no. The extrapolated results of no and EM, calculated from the linear fit through the data, are listed in Table 1.
The optical band gap and the average gap (EM) have similar behaviours with respect to annealing temperature, thereby implying that the growth of the native nano-crystallites and the nucleation of smaller crystallites in the amorphous network have similar effects on the band edges and on the conduction and valence bands. Therefore, the average gap can be used to describe the thermal induced changes in the optical properties of nc-Si:H.
The effect of isochronal annealing on the nano-structural and optical properties of nc-Si:H, with the emphasis on its relation to the hydrogen distribution and concentration, was investigated. Initial changes in the nano-structure are observed after annealing at 400 °C, as evident by termination of (=Si=H2)npolyhydrides from the grain boundaries caused by the growth of the native nano-crystallites. At higher temperatures, a further increase in the native nano-crystallite size and the crystalline volume fraction is observed, accompanied with the nucleation of smaller nano-crystallites and the subsequent removal of hydrogen from the amorphous network. At temperatures ≥600 °C the nucleation of the smaller nano-crystallites results in a porous material with an increased optical band gap and average gap, explained by the quantum size effect.
The authors acknowledge the financial assistance of the Department of Science and Technology, the National Research Foundation and the Council for Scientific and Industrial Research (Project no: HGERA2S) of South Africa.
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